How many Twitters/Facebooks/Dropboxes/Airbnbs don't exist because the founders sold them for $15M in 18 months?
Sometimes it's because the founders aren't really passionate enough about what they're working on.
Other times it's just financially insane to say no. A great example is Mint.com. How do you tell Aaron Patzer not to sell, when he's going to make $40M personally? That's more than he might have made in an IPO after many years.