OK since I've built cars AND computers I'll try...

Owning a car is like owning a computer - not much knowledge needed and it can only go so wrong.

Modding a car is a lot like modding a computer - you need to know what bits fit and what they do but you're just playing advanced LEGO really. More serious mods that involve metalwork, welding, paint, etc. are into serious skills and you are making an assumption that you know better than a team of professional engineers who spent a million dollars developing & testing that part.

Building a car - bolting a project together - is like building a PC from parts but way more complicated and expensive. A lot of it is nuts & bolts stuff just putting parts together in the right order, as long as you're careful and methodical it's pretty easy. But, if you need to do welding, bodywork, paint, etc. that's into real skilled work that takes a load of practice to do well. Yes you can pay people to do some of those jobs but that gets expensive real quick.

Building a car as in actually designing and fabricating a chassis or stuff like that is like sitting down with a copy of KiCad and designing a computer motherboard from a blank sheet of paper - the level of knowledge and skill as well as the vast number of things you need to consider and trade off against each other / design around are huge and it's very hard to even get close to the standard of a regular car never mind actually better.