In my experience, codebases never maintain their level of quality. Unless you are taking deliberate action to improve the codebase with every commit, it's actively getting worse like you described above.

The interesting thing is that this isn't a technical problem, it's a human problem. Most folks want to clock in, do their job and clock out, and going the extra mile to improve things isn't worth the effort.

I think its the same human tendency that leaves so much pee on the toilet floor - some humans will clean up their own, but almost none will clean up someone else's.