Jack of all trades, master of none. I also somehow doubt that you've built a car from scratch, including designing the engine, carving it out of a block of metal and so on. And if we're talking modern car, good luck fabbing the integrated circuits in your backyard or whatever. Even your particular generalist fantasy will (and most likely has) hit the hard constraints of specialization real quick.

There is no single human alive that can understand or build a modern computer from top to bottom. And this is true for various bits of human technology, that's how specialized we are as a species.