Because it's not portable. The world we find ourselves in is at least x86 and ARM, and assembly for one isn't good for the other. LLVM has a hardware agnostic intermediate representation (IR) that one could use though.

you're not thinking vibe enough. Your prompts are now the program, that's your abstraction. Ideally, if it would work, LLM would be the one then producing each platform's output. Maybe even compiling from higher languages to gather some compiler optimization hints and then doing full salvo of asm. Ideally.