In brief: Most venture funds are closed-end funds, with a certain amount committed up front from LP's, where each dollar can only be invested once. The fund stops making investments after 5-7 years, then closes down entirely after 10 years. After 10 years, the managing partners stop collecting their 2% management fee, and any additional returns aren't typically included in the performance of the fund.
As such, there's a huge incentive for fund managers to invest early, and find an exit within the time horizon of their fund. A VC who pumped a ton of money into an early stage Uber in 2014 in year 6 of their fund, is going to be pretty screwed if Uber decides not to go public for another 5 years from now.
Where this could go sour is that these same fund managers are spending years 9-10 passing the hat to raise money for their next fund. If the typical pool of investors still have a lot of money tied up in previous funds, then they're going to be less likely to ante up for the next fund, since they'd have to double down on VC as a proportion of their portfolio. It's also going to be a tough road show if your story is that you invested millions of dollars in your last fund and it's all still tied up ...