The thing is that this is a private company, so there is no public market to provide liquidity. The company can make itself the sole source of liquidity, at its option, by placing sell restrictions on the grants. Toe the line, or you will find you never get to participate in a liquidity event.
There's more info on how SpaceX uses a scheme like this[0] to force compliance, and seeing as Musk had a hand in creating both orgs, they're bound to be similar.
[0] https://techcrunch.com/2024/03/15/spacex-employee-stock-sale...