Machine code isn't especially expressive per line or unit of code. Lower level languages takes up more of an LLM's context than higher level ones.

To be effective in using low level languages, LLMs would have to build higher level constructs like subroutines from scratch every program.

It's not that different from why we almost never use assembler for anything more than code islands: even a modest subroutine can overwhelm our own mental context window.