Also, there are jobs within the nursing profession that don't carry the same risk as working in a hospital, taking care of Covid patients. Plenty of nurses work in private doctors' offices. In fact, virtually every doctor has a staff of nurses, usually more than one.
Of course, they could be exposed to Covid that way also, but obviously much less exposed than working in a hospital directly with Covid patients. They might encounter a patient with an active case of Covid that way, and usually won't even let them get in the door. They take your temperature out in the hall, or the minute you enter the waiting room. And they probably come in contact with someone who has it once or a couple of times a week, not all day, every day.
There are also nursing jobs in labs, schools, corporate offices, etc.
I don't think money is the only, or even the main motivation for keeping their jobs as long as possible. Most of them seem to regard it as a calling. They are the true heroes of this disaster.