Theoretically, as the CEO of Quip, he has some interest in helping his coworkers cash out too.
Obviously the rest of the team is likely to be very talented as well. Also, he's the founder, which includes a responsibility to act in the best interest of the company (plus $20M/yr would be much less money).
Also, someone like Bret is very unlikely to be attracted by purely monetary gain like a $20/mil year salary(or even $50/mil year.) He probably cares as much, if not more, in being able to achieve some larger goal--which requires building a sort of team he has already built.
Nothing like abandoning a company you built right as it hits its stride.