The boss can't hire new grads to work on it, because they can smell the manure from a mile away. So as long as you're willing to slap on yet another coat of superficial patches, they'll keep paying you to do it.
That's an equally-bad symptom of the same problem, approaching from the opposite side. The structural problem still doesn't get fixed, because the developers aren't repulsed by it at a visceral level quite enough to kill it with fire or nuke it from orbit. It's just scary enough to scare off the competition.
The technical debt is kept in place, so that people can live off of the technical interest payments. It's not an honorable living, but it is a living.
Every, and I mean every, development shop has some amount of tech debt. But even if the code is generally good quality, you have a subset of devs who think maintenance is below them. That's a bad attitude and people don't want to work with that.
I've seen a couple fresh grads come in with that attitude and get away with it. I think it is more personality type than anything. What I saw usually happen was they were allowed to work on a new feature as kind of a treat for joining, but after when tasked to start fixing issues from legacy, they flat out refused. Even taking it to the head of engineering and making a stink.