[bcherry]

RSU grants given to employees (including large grants given to new executives and key employees) are typically illiquid until 6 months following an IPO. The company has to IPO to make good on the compensation given to hire and retain their employees. If they never go public, employee compensation is worthless, and they cannot hire nor retain the best people.

[encoderer]

Most often pre-IPO companies grant options not RSUs. And the lock-up period is a post IPO lockup designed to stabilize the price of a newly offered security while it finds its market.

The existence of a lock up period does not inhibit pre-ipo trading on the secondary market, though to be clear there is no truly liquid "secondary market" for most companies.