Same reason anyone stays in a war torn land. It's your home. Your land. Your world. You'll be damned (maybe literally in this case) if you're going to let someone push you off of it. You're either going to keep it, or be buried on it.
Some people are too poor to go to other city states(or are wanted criminals in those),some are unwilling to let the undead take their stuff.
And don't forget that in the lore,the harrowing doesn't come at the same day,there can be multiple harrowings in a year,or there can be years without harrowings,so it is not like the people get a deadline when they should leave.
Why do people live in Tornado Alley?
If they don't fight, it'll just spread farther inland