Publicly traded stock gets deposited in your brokerage account because your employer is publicly traded. Private companies offering double trigger RSUs that don't turn into shares or cash until a liquidity event (I.e. an IPO). At that point (plus a lockup period, maybe, depending on how the company goes public) the shares, minus withholding, will get deposited into a brokerage account and be available to sell.

(Disclaimer: I am a holder of Stripe RSUs)