People might have more than one reason that they are at a company. Many people have a job because they do indeed want and need money, but they have their particular job because they like the work there more than the work at other jobs they may have taken.

If it was just about the money they would have probably left. They are well known enough to almost certainly be able to quickly get another job making at least as much money. Add in the 6 months severance they would have gotten and leave is the money maximizing option.