They generally don't want their cap table to explode with randos, and if there's access to sensitive information, they obviously don't want the cases where unscrupulous party buys shares just to feed that data to competitors.
With that said, it's obviously in their interest to provide some liquidity to avoid their long-time employees from defecting to GOOG or NFLX or FB, which reward with perfectly liquid stock grants, so in case of demand from the buy-side an employer would orchestrate a secondary market transaction. On the buy side in most cases you'd see an SPV managed by the VC who invested in previous rounds (which helps with keeping the cap table low).
Ironically, for smaller VCs entire economics of their firms are based on these SPVs (which sometimes charge upwards of 2% management fee on top of 20% carry - and that's for a chunk shares sitting quietly doing nothing).
With that said, it's obviously in their interest to provide some liquidity to avoid their long-time employees from defecting to GOOG or NFLX or FB, which reward with perfectly liquid stock grants, so in case of demand from the buy-side an employer would orchestrate a secondary market transaction. On the buy side in most cases you'd see an SPV managed by the VC who invested in previous rounds (which helps with keeping the cap table low).
Ironically, for smaller VCs entire economics of their firms are based on these SPVs (which sometimes charge upwards of 2% management fee on top of 20% carry - and that's for a chunk shares sitting quietly doing nothing).