The answer is cost. We used to have the computers you're describing. They were made by SGI and the hardware & OS made all sorts of guarantees like the ones you're describing, to the point where XFS was ported to Linux x86 initially it had all sorts of bugs that simply couldn't happen on SGI machines.
The x86 boxes were cheaper, nobody really cared enough about hardware reliability enough to pay the price for the likes of SGI, and the rest is history. Today we have a "worse is better" hardware architecture and software has to be able to handle it.
The answer is cost. We used to have the computers you're describing. They were made by SGI and the hardware & OS made all sorts of guarantees like the ones you're describing, to the point where XFS was ported to Linux x86 initially it had all sorts of bugs that simply couldn't happen on SGI machines.
The x86 boxes were cheaper, nobody really cared enough about hardware reliability enough to pay the price for the likes of SGI, and the rest is history. Today we have a "worse is better" hardware architecture and software has to be able to handle it.