If you've never or very rarely picked up a book you think you'll like only to be disappointed you're very lucky indeed. As for continuing a book that is falling short, I'd say it's quite common for many reasons. Bought it and don't want to feel as if it was a waste of money, nothing else to read, everyone else loves it so there must be something in it somewhere that you'll eventually like, discussion/book club/education purposes, a single compelling plot point hiding amongst the drivel, interesting structure/literary technique that you want to see how it's used despite not liking the story, sunk cost fallacy and an aversion to DNF off the top of my head.

All that about ratings may sound reasonable at surface level, and it's how most people use rating systems, but don't you see how it lowers the range of ratings thus making them less useful? If you don't like a book you give it a 2star, but 1star was literally made for books you don't like. It isn't for books that are an affront to humanity that shouldn't have been published. And while you only cut off 1star it should be easy to see how others cut off 2star as well in the same way making it even worse. If everyone decides for themself what the star means for them personally instead of using the defined meanings they become useless. This post for example uses 3.5 star as an example for a terribly rated book that most people would avoid, whereas to me there's absolutely nothing wrong with a 3.5. That's almost 4 which is a very good score.

I think it boils down to many if not most people thinking 1star means it was a bad book so they don't use it. That's not what it means. It just means you didn't like it and there's nothing wrong with that.