Could you share some examples of that occurring? To my knowledge, a lockup period is almost always a standard requirement from the underwriter (on any larger IPO), and in some states is even required as part of their Blue Sky Laws.

If a founder/VC wants to get out before the IPO, they usually just do so on the secondary market. Example: Benchmark cashing out a portion of their Uber stake to SoftBank pre-IPO.

https://www.sec.gov/fast-answers/answerslockuphtm.html