You retain employees who might have been great from the zero to 50 stage but not as well suited in the 50 - 5000 stage. But their incentive is to stick around or give up potentially millions in equity that they busted their asses to earn.
Checked out employees aren't doing anyone any favors. You might say you can fire the person or put them on a performance improvement plan but this is easier said than done - especially if it was a key early hire.
I've seen this in many SF-based companies.