Because people are not saying "let's replace Casio Calculators with interfaces to GPT!"

By and large, the processes people are scrambling to place LLMs in are ones that typical machines struggle or fail and humans excel or do decently (and that LLMs are making some headway in).

There's no point comparing LLM performance to some hypothetical perfect understanding machine that doesn't exist. It's nonsensical actually. You compare it to the performance of the beings it's meant to replace or augment - humans.

Replacing non-deterministic black boxes with potentially better performing non-deterministic black boxes is not some crazy idea.