This isn't about stock options or public markets. These kinds of companies are owned by the employees. You or I can not buy stock in them. This can be a much bigger incentive than regular options or the toilet paper options startups offer. Payout is over several years after you leave, so people care about the long term rather than next quarter. From your comment it seems you're not at all familiar with this model. It's not perfect but it's way better than stock options.
But that means employees cannot sell their stock, either. So if one of those millionaire employees retires and the company goes bankrupt one year later, the million is worthless all of a sudden.
