Some of my colleagues are still here because they cannot find a better option, and some just because they are waiting for a package. I think that waiting for severance pay is a terrible excuse for why some are still here, because - how sure are you that you will get the package at all?
Someone mentioned financial collapse happening here and that's something that should be kept in mind. It's hard to give advice to people because everyone knows their own situation best, but I would always recommend everyone who gets the chance to leave - if the offer is solid, do it and do not delay leaving.