If early liquidity wasn't an option for founders, this massive pre-ipo/private-ipo market wouldn't exist.
Without early liquidity, a pre-ipo zuck/kalanick/etc. would be a paper billionaire with $0 in the bank and >$1bn "invested" in a risky tech startup. That's not financially or mentally sound, even by their risk lovin standards.
If investors demanded every penny go towards growing the business, those CEOs would just IPO earlier to get liquidity.
It's a necessary alignment of interests, if investors' interest is delaying the IPO.
Employee options holders can't just decide to take a company public, so their interests can stay misaligned.