Why do AI coding tools love copying your worst legacy code?

It's funny how smart these models are, yet they will completely ignore the clean patterns right in front of them and mimic some weird workaround I wrote in a utility file two years ago.

The more context or files I attach the more it hallucinates random functions that don't even make sense for what I'm building today.

I'm starting to think manual context curation is the only way to keep the code clean. How do you feed your tools the right files without overwhelming them with noise?