Perhaps because these movies were mixed for theaters with very specific digital sound setups and now lots of people see them for the first time at home.
To be fair, the problem of matching to soundtracks to people's real listening environments is not entirely solved. That is, back in the DVD a lot of DVDs had a weak 5.1 mix because it was designed to sound OK if you played it back on a 2-channel system so you wouldn't really use the center channel. Then in the Blu-Ray age they got more aggressive with the 5.1 track at risk that you'd miss the dialog if your settings weren't right.
So on top of movies that aren't legible in the theater there is plenty of trouble that comes up in the mixing for home theater.