A founder who believes in his business, will never cash out early. period. He would always get more after an IPO or exit. So why would he? I assume the founders are not stupid, they know their valuation is not justified.
What? It's a whole lot easier to swing for the fences when you're already a millionaire. Selling 5% of your total shares for $3,000,000 sets you up well for life. Not great, but well. At 4% interest that's $120k/year. After that, it's gravy.
On Wallstreet they call this "fuck you money".