Because job hopping is scary as hell (especially for developers who struggle with imposter syndrome) and job hunting is generally shitty.

What if I don’t like the new place? What if I can’t feed my wife and kids? What if I’m actually terrible at this and my current place is so stupid they haven’t figured that out? What if the economy tanks in the next couple of months and I’m out on my ear with no severance pay?

Better to stay put, accept slightly less money for another year and look at it again when I’ve got the time and energy to cope with it.