I have a similar problem, but they are actively making new messy code.
When basic things are not being done right, it feels almost demeaning to try to start a conversation with someone that the variable name "stuff" might not be descriptive.
I was in the same situation. It's bad working with poor code, but it's ten times worse having them pile on shitty code every day. And oh my, was it bad. There was no code reuse, if they needed the same sql query in another place, it would just be copied over. Or if there were two slightly different queries based on some parameter, there would be an if/else statement and two separate queries - often with embedded variables (hello sql injection). Routing was one big switch/case statement. Commit messages would often be non-descriptive, like “chk”. Frontend was a special hell, jQuery spaghetti code, mostly single page, with multiple levels of nested ajax callbacks. I tried to be patient and teach them, but the bad practices continued. I even managed to rewrite parts of the backend with a framework and an ORM, but the original devs kept writing code their way. At some point I just needed to protect my sanity and left.
When basic things are not being done right, it feels almost demeaning to try to start a conversation with someone that the variable name "stuff" might not be descriptive.