Has anyone else started a new job with the promise of interesting work, just to end up spending all their time writing e2e tests for crap code written 5 years ago by a contractor who clearly couldn't give 2 shits?

If they are paying you to not work on money making parts of the product, they're either really big, something bad happened recently or you're not charging enough. That's the most common reason that they don't happen + if you want good quality, it needs to be tested by humans regardless.

TLDR: The previous contractor didn't care because the client didn't care

Yeah, I've worked job where the most interesting part was the fucking FizzBuzz test I had to do

Not a single one of the jobs I've had was as advertised