While yes indeed some employees have waited many years for a liquidity event and want a payout for their hard work, the other shareholders are LPs with a venture capital target date on their funds.

This means that the company will either need to keep raising money to buy out earlier investors that need to hit their fund target dates (Rather than raise it all for themselves), or they'll need to go public at some point, even if they don't need the money per se. Keep in mind that investors get lot's of sway via their board seats.

If you're in the business of losing money, this game of hot potato becomes harder and harder to play over time.

(Someone correct me if I'm wrong.)