"Medieval" isekai: why a horse-driven carriage instead an armored all-terrain vehicle?

If you had the magic and the resources can you build a functioning armored vehicle?? Well... some MC can like
Arifureta don't know how and don't care Black Iron’s Glory he tried
Manuke FPS he cheated
Release that Witch he was a mechanic, engineer think  If I end up in a other world situation, all I can do is grow trees and hope to be a druid. Hell, I can't even create a pump and make a revolver so let's forget about the TANK and  APC stuff.
BTW the Lazar V gives me the feeling of Hammerhead ??? I think it was called from MassEffect 

+1. If you've got the strength and ability to go "F'it, i'm gonna do this properly" then it'd be stupid to not do it

If the protag has a reason to conceal himself (like he's not yet at the stage where he can tell countries where to shove it), sure it's best to keep a low profile. What's stupid is when protags have the legit power to wreck the world but act like a weak protag, that's where it gets stupid.

TL;DR: It's situational. If you've got the power to wreck the world and frak anybody who might oppose you, then ditch the medieval shit and teach them the gospel of modern warfare. Modern Weapons Cheat in Another World is a good example of how to do it properly.
If you're not strong enough to openly do it, then get creative and do it on the sly. 

That is what the Mc of Magicraft Meister did.
Of course he also build speed boats, and planes, but he made a carriage with golem horses, and upgrades like a proper suspension for traveling since he is hiding just how much above the average nation his ability is (getting horses movement right was major enough).

This is the thing that needs to be considered. Automobiles are not held the same in every country, and many do not understand them well enough to make and fine tune one. Most would understand the basics of the engine, but without a similar fuel that is useless and you would have to rebuild almost everything from scratch.

Outside of crafting series where the Mc gains insane fabrication abilities (like Arifureta, Magicraft Meister, etc) most MC's do not really have the knowledge or skills to make an engine and transmission.

Plus if the setting is still using horse and carriages then that usually means there is some problem with making a pure magic vehicle, as if they know how to make magic items, then that is something they should be able to figure out on their own that the MC might not immediately solve. 

The magic system in Release That Witch is screwed. MC had to design steam machines and steam turbines. Magic is only used in production.

Just make the thing and use magic to move it. No need to complicated controls or differential or manual transmission and sh*t. 

Me too lazy to change me transportation. Me just put more pillows in carriage 

Hey ossan, nice! 

Can YOU build these? Ai-chan can't, even if Ai-chan has access to superior materials. Ai-chan doesn't even know how these vehicles work. Unless you actually work in the automobile industry, these things are like magic that the average people does not understand. Most drivers don't even know how to change their tires, much less make these vehicles.

This isn't even going into the realm of fuel yet. Do you know how complicated fuels actually are? Synthetic fuel can be made easily, sure, but is the synthetic fuel you make suitable for your engine? Will it run smoothly or will it stall, clog up or just explode? If you get sent to isekai and you want to build all these yourself, not only you need to know materials science, you also need to know organic chemistry, mechanical science and smithing. It took our scientists almost 100 years to create fuels that are suitable for our engines, through experimenting with adding organic oil, ethanol and lead.

If a single person can do all these, that's a Mary Sue, and that is bad. Because that's not a genius, that's a god. If you know anyone who can fulfill all these roles in real life, start worshipping him.

Now, sterling engine carriage, THAT is doable. The concept of sterling engine is pretty simple, and as long as you have the materials and the ability to create differential temperature (fire or ice magic does the trick), you can make a sterling engine carriage. You can add the gears and breaks as you go along, but the concept of a sterling engine carriage is buildable by medieval standards. The only problem is, sterling engines are kind of heavy, so the materials need to be light and strong, which in medieval standards, means magic materials.

This is a sterling engine car. Not too bad, right? There's just one problem, the materials to make this car did not exist (or were extremely rare) in Europe in the 16th century.
 

The hose is used to whip the slaves pulling his carriage. 

Except the argument that they keep coming back to is "because magic" as a reason to ignore every reason, cost feasibility WAS brought up before by the way, not just here but in a LCD thread the OP asked it in before posting this.

Aircraft carriers are on the line of cargo ships, this thread was on stuff closer to luxury cars. To be more specific the question was why in a worlds were medieval european transport vehicles are the best available ANYWHERE does the MC not just make a (armored) magic car instead of just improving the vehicles available.

What I have been saying is that there needs to be both a reason why the MC can make such a vehicle, and why the rest of the world does not have any, which only makes sense when the MC has a cheat like in Arifureta (why the rest of the world can not make one is explained well before he acquires the skill needed to make his hummer) or most of the knowledge is there but a key piece is missing (which requires magic and materials alone to not be enough). Keep in mind that with a magical power source that can be used to rotate a wheel, you could make a vehicle with medieval level knowledge of windmills as a basis for a transmission and most series are set in a society developed well past (ie : several centuries) the point windmills, crank shafts and even pistons were developed, and simple steam engines existed in Ancient times let alone the medieval, and magic materials would likely solve allot of the issues that they would have in being used as a power source.

Costs involved is one of the reasons brought up as a potential reason why not.
Other potential reasons include :

The settings level of magic is to low for such a vehicle to work.  the time involved to develop and fine tune such a vehicle could easily exceed what is available to the MC (in the thread I mentioned before the MC has less than a month to make a vehicle, and has never made anything with parts moving by magic, but went overboard on the rest to the point the local lord saw it, asked how much it would cost to make one, and placed an order)knowledge, very few actually have all the necessary knowledge of how the engine, transmission, cooling and everything else in a car works. needing to blend in and not stick out to much (these could paint a big target on the MC's back, and in most series they are not actually invincible, immune to assassination or strong enough to take on the whole world)potential fuel efficiency (which is related to cost), does the vehicle need some form of fuel, or can it recharge and if so, how long can it run vs how long does it take to charge? There are some series where you see vehicles and mecha that can operate for short periods of time, and require long recharge delays. The nature of many magic systems does not lend itself to these types of creations (seriously in many series the nature of the MC's magic of skills does not mesh with making a magic vehicle)

 

All you need for transport is a can of magic beans, travel with the power of farts. 

Personally, if you're going for the ultimate magic transportation. Just get an Air Ship.... 

coming back to this thread because of a like notification, i'm amused at people seeming to miss the point and looking at the problem realistically. This involves isekai and when has that ever been realistic?
OP also mentions having magic and magical materials.

The arguments, when looking at the idea realistically, are sound but that's not what the question is asking.