Who said anything about quitting before getting a new job?Unfortunately the days when someone could quit their job and find another next week are long gone.
If you really want to, you could get a new job anywhere, while still in employment; there are plenty out there. But it's the hassle of chaning industries (perhaps) or a pay cut that people wouldn't like.
It's just weighing up the options. Rarely is there only one option. Would I quit my current job before finding another one? Hell no! But if the company was really that bad, then I'd be looking and take nearly any other sustainable job.
And that is the problem. People want to have their cake and eat it. If people were more free about leaving, companies would do more to keep them.