If too many of them do, the stock goes down, and essentially everyone at the company (especially the executives) take a pay cut. The executives aren't even just thinking about their own pay. The entire company's ability to attract and retain talent depends on the stock price going up.

If a too good offer comes by, for instance, for one to buy Google:

the offer has to be disclosed; and regardless if the founders like it or not -- or what the voting majority says, there is a price where the remaining shareholders can force Google to be sold.