Make is a great language, but very few people know it, or care about knowing it. The vast majority of makefiles are automatically written by garbage such as automake. They are exactly as you described - reams of repetitive nonsense. That's going to be the training data for the LLMs, so no wonder they write bad makefiles.
Yeah, that makes sense I guess. I'm used to small hand-written Makefiles for projects where people actually care about being reproducible. I'm not a fan of automake although I have used it.
Just for shits and giggles I decided to let Copilot (whatever the default in vscode is) write a Makefile for a simple avr-gcc project. I can't remember what the prompt I gave it was, but it was something along the lines of "given this makefile that is old but works, write a new makefile for this project that doesn't have one" and a link to a simple gist I wrote years ago.
Fuuuuuuuuck me.
It's 2500 lines long. It's not just bigger than the codebase it's supposed to build, it's just about bigger than all the C files in all the avr-gcc projects in that entire chunk of my ~/devel/ directory. I couldn't even begin to make sense of what it's trying to do.
It looks mostly like it's declaring variables over and over, concatenating more bits on as it goes. I don't know for sure though.
I won't be using it.