The Postgres query planner has had to operate, for those same decades, in a much more realtime-sensitive and restricted environment than compilers. It can only draw its conclusions from summary statistics on tables in isolation, not on their relationships with each other (and even less so when filters are involved). For many cases this is fine! For many others it isn't.

There's a good number of heuristic choices in compilation where, maybe, you could get more optimal outcomes with machine learning - but at the cost of compilation resources, both time and space, and possibly determinism too.

As an example, register allocation is graph colouring, and thus NP complete; a model for producing an allocation plan is learning heuristics that might look at more features in combination than the ones hand-crafted into the compiler. An LLM for the job might do better than a more focused model like a GNN, due to sheer size, the effectiveness of transformers, or magic. But it probably won't do an overall better job than the handcrafted heuristics, because those handcrafted heuristics also tend to compile very, very fast with a small memory footprint, and can be debugged (more) easily when they go wrong.

I think there's a future in which LLMs are used for auto vectorization