AND IF IT IS EASY THE LOCALS WOULD HAVE IT ALREADY. Stop coming back to "because magic" because that is not a reason or argument, and the nature of the setting negates it because if it was simple someone would have figured it out and the knowledge would have spread by either trade, or more likely conquest. Look at how much land mass Genghis Khan conquered with horses, now give him motorcycles and try to figure out how much more land he would have taken.
If magic existed in our world then the Romans would have made some form of magic tank (if no one else had done so before hand), let alone 16th century europe. Or the chinese would have, or any number of other ethnic groups. There were many in history that were great engineers, and one of those empires would have figured it out.
Either that or they would have made some form of flying vehicle like the airships you see in to many settings, which make the whole car obsolete from the get go. If I was sent to a magic world, and there is magic was a sufficient reason to be able to make anything, I would be working on some form of airship, not a car much easier as it is take an existing vessel, add levitation and some fans for propulsion and there you go. maybe adding some form of invisibility, and pressure controls for later on.
The question is not why does the main character not make these vehicles, the question you need to answer is why has the setting not made some form of vehicle already. Unless the setting is written by an idiot there will always be a reason somewhere be it there being a lost and/or cheat skill the MC has that is needed (Arifureta is a perfect example), the costs in materials, production fuel and/or maintenance makes them prohibitive, the world is recovering from some tragedy and has not rebuilt their knowledge of magic to that point yet, the people in power have a superior method already like teleportation and as such have no need to a car, so are not funding the development (if say magic is focused on the nobility and hoarded, it could easily be a case where there are have and have nots with the haves not having a need but being the only ones with the means). In most of these worlds the magic system has rules to it, and often those rules actually are not that conductive to making magic tech, which is common in allot of systems focused on elements, heck there could even be suppressed knowledge or religious doctrines where making a car would paint a huge (blasphemer) target on the MC's back (and contrary to belief, the MC is often actually not strong enough to take on the whole world, a notable portion yes, but not the entire world, unless the previously mentioned bad writing comes up), etc.
If all the main character has is I have seen them, that is not enough of an advantage, because the populace has seen similar enough befor eas well.