Don’t understand people who are very well read, but still not very good readers

I’m not thinking of anyone in particular as I write this: it’s just a stray thought I had awhile back. Have you ever noticed how there’ll be people out there who read tons of book–they read constantly–but they still seem to be kind of undiscriminating? Like, they’ll praise stuff that’s bad and be turned off by stuff that’s good? I don’t get it. This seems almost willful. Like, I generally thought that an appetite for good writing is something that arises after you’ve consumed tons of bad writing and gotten mightily tired of it. But that is not always the case, for some reason.