It's just the way human mind works. It's easier to just say "yeah that's great" rather than give some actual feedback or criticism. Why do you think "critic" is an actual profession?
It's just the way these rating schemes work in general... maybe it's that sort of individualistic culture where we've used to getting trophies from the tiniest, insignificant things... like, hey, you participated, here's a trophy... here's a trophy for good effort... and here's a consolation prize for the loser... so eventually, getting prizes loses meaning. And we've come to expect praise from all of the things we do...
Thus most people are just going to default to giving good ratings at least when they don't have any real complaints that would justify giving a poorer or average rating. 5 stars is seen as the default baseline, and any lesser rating requires for there to be something wrong to criticize.
5 stars is maybe just too granular for most people to grasp. Not because people are stupid or anything. Just that most people aren't going to put that much effort into evaluating "hmm, is this bass solo worth 3 stars or 4 stars"... it's just easier to say "I like it" or "I hate it" than to give an accurate point rating without any kind of scoring framework. And sure we could come up with some kind of framework but there'd really be no way to enforce it because all ratings are subjective and if we take that away they lose all meaning.