Hmm, interesting questions…
I don’t often not finish a book, although I’m pretty picky about what I read in the first place. That is, I tend to mostly choose books by authors I’ve read before, or who are highly recommended by people or sources that I trust.
But I do agree entirely that if you are not enjoying what you’re reading, then you should stop. Life’s too short to read bad (or even mediocre) books!

I posted an article recently on why I read. You can find it here:
https://medium.com/@tomwaterton/why-i-read-d044f8a01015