Modern CPUs hide so much logic that the assembly language is an abstraction itself. CPUs have many times more registers than their assembly language, execute instructions speculatively and out of order. Multiple layers of caches are synchronized in complex ways. There's an invisible complex work scheduling algorithm that can even make one CPU core interleave work of two (hyperthreading).

GPUs can expose more of their internals thanks to shader compilation. They don't have to emulate previous-gen chip, and instead every chip can expose exactly what it supports and rely on software being recompiled for it.