For example: On an IBM PC, running DOS. You could know all there was to know about the BIOS, hardware ISRs, DOS ISRs, DMA, how a file system works, and maybe a bit about the graphics system, and you could make the machine do anything you wanted.
Today, if my kid wanted to learn all those things, I wouldn't even know where to tell him to start. You simply can't get that low (easily) anymore, or know that much about numerous subsystems. And the higher level stuff, like you say, is buried in layers of abstraction.
Of course, now we have an unprecedented access to cheap, powerful microcontrollers and SBCs, which make up for it to some degree - but it's still harder to translate that knowledge into something of perceived usefulness on a 'proper' computer.