People have given a lot of good reasons, but there is one other that I wanted to add, with an example.

For Project Hail Marry, Andy Weir got some really excellent quotes for his book. One of these was from George RR Martin. Which was kind of a big deal for him, and everyone wanted to use.

The problem is that Martin did exactly what you mentioned- he talked about something he liked about the book. The thing he talked about was something that Andy had been trying not to spoil about his book. So this created a problem- give away a major plot point, or not use the quote.

The more a book's quote say about the book, the more it has the potential to spoil. For instance, if I say a book is 'really, really good' that may be an endorsement. If I say 'Man, I didn't see the third act twist coming, and it really recontextualizes the writing before that point in a way I find interesting,' then some readers might find that enticing, and some might be pissed that I just gave away the fact of that twist.