I think that it often works in reverse: the acquisition fails, causing the founders to leave too soon. I don't doubt that some founders see the $$$ and can't wait to leave, but there are lots of founders who genuinely want to thrive at the new company. Sometimes the acquired startup doesn't get the attention or resources it needs to successfully integrate into the new company, or maybe it gets put on a back burner. That may cause a founder to want to leave.