PCs are one of the few things we build ourselves because it's one of the few goods that have standardized and commoditized parts.

If there was a large degree of interchangeability between engines, transmissions, bodies, dashboards (etc) the auto enthusiast community would for sure be building cars from scratch out of parts. But realistically the pieces are tightly coupled and you can't pick and chose.

It's the same with coffee machines - if there were interchangable pumps and boilers and group heads etc, I bet building your own coffee machine would be the norm in a certain crowed.

And to be clear there's good technical, aesthetic, regulatory and business why most large machine's are made of interchangeable parts. I'm not saying car and espresso machine manufacturers have done something nefarious. Just that PCs happen to be free of a major constraint.