Cars are too simple now and you couldn’t fix one even if you wanted to since they are so locked-down.

I mean, I'd argue they're too complex. But I agree, you need so many specialized widgets (many that vary by brand and model) that its impractical to do more than change the oil.

I was looking at a Model A on display at a dealership when I went car shopping recently. They had the engine open, and I was looking at the thing thinking "If you sent me this in a box as a 'Build your own car' kit, I'm pretty sure I could do it".