> I think it's simply that modern machines are (generally) so reliable that you never have to get under the hood to figure out what broke until you (rarely) do and then you have no conceptual framework for trying to resolve what is broken.

You have the same problem with cars. When I was 18 I was able to disassemble and reassemble my car and basically fix anything which broke. I even replaced the bodywork one time after a crash.

Today nobody even changes their summer-/winter-tires on their own anymore, let alone changing oil or renewing breaks.

You could also say that the machines became so reliable and so complicated that most people don't need too look under the hood and if they need it's too complicated anyway.