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

bad example because it's not that other countries do have the need or cost to build one, but that they lack the technology to build one

also the US only has 20 aircraft carriers in operation right now and as for the rest of the world, it is the nuclear engines that is preventing them from building their own

not to mention that all of the world powers will go cra cra on you if you try to build anything related to nuclear 

Actually that is not totally true even in our world. Need, support cost and manufacturing capability also play a huge part. For example, if you want to compare totally top of the line iconic vehicles, an aircraft carrier would be the closest analogy in our world but while the US might make those things by the dozens, many other countries in the world don't find the need or the cost to build one efficient.

Another example would be even when we have air freight, goods are still shipped mostly by sea.

Horses would still make sense in a rural area if the cost of supporting an airship was $40,000 per flight hour or they can't get their "magical" fuel easily or that they can't make those things enmass. Even today there are still rural areas that use horses since they are the ultimate in "self fueling" transport.

But frankly if it is a story? Well, because plot. lol. 

realistically
it require :
~ knowledge
~ access to resource(material, fuel, local world technology expert or something like that)
~ fund
to create and maintenance at least those three must be cleared~ well if using cheat then go ahead
pretty simple

now move to effect if someone make such thing~
he/she will become target from various faction because posses such technology unless somehow big shot ignore it for whatever reason.......... duh~ if this cat was someone on power this cat will make various move to posses such technology~

it is bothersome~ 

Noted. Edited. Thanks. 

thats with modern suspensions on the wheels and on an even road and its only for a while. imagine bumpy offroads for like a week straight while sitting on the equalient of a bench(the more high class noble ones would prolly be comfy tho) 

I think it would be dependent on certain necessary materials that might be extremely rare or you have to tune a lot of magic items together. Then we also have to consider getting hands on magically attuned metal and other stuffs. It would require a lot of know how as well as knowing what would happen if they worked together. 

Because when we ignore realism, or more specifically what would make sense in the world of the story, the only remaining answer is a bad writer and incompetent world building (which is way to common, especially in the bad self insert power fantasies) and does not deserve any discussion.

Magic and magic materials does not explain why the population dose not use them already, and in fact there are plenty of series where they do have such vehicles or better means of transport just look at the final fantasy series where the iconic transport is a flying ship, because thinking that they know how to easily make golems and the nobles and major merchants still use biological horses instead of magic ones as a status symbol is quite frankly idiotic, especially if it is a high magic setting which is common and kind of necessary for that reasoning.

Realistically, if  any assumption about the nature of magic and  materials available is something that would have to be applied to the setting as a whole since it would affect every aspect of how that world developed, and why that area has not been conquered by someone else who did developed these measures if all it takes is 1 person with an idea and the materials are available to the commoners. 

That part is under "manufacturing capability" or lack of.