This is not necessarily true. Many models have been trained on assembly code, and you can ask them to produce it. Some mad lad created some scripts a while ago to let AI “compile” to assembly and create an executable. It sometimes worked for simple “Hello, world” type stuff, which is hilarious.

But I guess it is easier for a large language model to produce working code for a higher level programming language, where concepts and functions are more defined in the body that it used to get trained.