The fact that employees are willing to cash out now might mean that they are less confident about the potential of cashing out big in the future. I'm sure no early Google employee would even think of cashing out pre-IPO.
It might. Or it might just mean that some employees want to buy houses, new cars, etc.
People are very good at "doing without" when they're starting a new company, but there's a point where life catches up and they want to start living in a bit of comfort. This goes double (or more) for those people who have families -- I'm not sure quite how one would explain to a 3 year old that they're not getting the toys they want because daddy wants to hold on to his stock options so that he can become a millionaire when facebook IPOs a few years from now.
For instance, if I could stop working tomorrow, and maintain my current lifestyle, I would take that over promises of more cash later. I might do this even if the payout were not enough for me to fully retire on.
As long as I can take an ~5 yr sabbatical with no strings attached, I'm confident I can create enough things to keep the money coming in, on my own terms.