A significant number of PhDs drop out, and likely literally everyone undergoes some sort of huge realization of how doing a PhD is completely and surprisingly u

A significant number of PhDs drop out, and likely literally everyone undergoes some sort of huge realization of how doing a PhD is completely and surprisingly unlike what they expected (which is still true even with all the advice telling people that doing a PhD will be surprisingly unlike what you expect). Someone dropping out of a lab is a significant loss in time and resource investment which labs would prefer to avoid, and the biggest predictor for staying in a PhD is substantial experience doing research (to have some sense of what the lifestyle is like) and little experience with higher paying jobs (here at MIT, I see many PhD programs prefer people straight out of undergrad, even though older people are orders of magnitude more productive in the early phase of their PhD).