Most people know that if a car share driver gets enough bad reviews they get kicked off the platform. Maybe part of that trend is not wanting to be responsible for "taking away" someone's source of income, even if the service was just average. Really egregious things can be reported in other ways.

There's also the cognitive burden of having to compare "this" ride to "every other ride you've ever had". That, plus the fact that you rarely see an impact of giving reviews... it's easier to just hit 5 stars or ignore.