I just realized:
While manga tends to avoid internal monolog generally speaking, anime is just chock full of internal monolog. And (in poorly done anime) when there isn't much use of internal monolog there's a tendency to do exposition outloud when it would make more sense for the characters to play it closer to the vest.
It's funny, I never noticed that before. Although the narrative boxes in manga often contain the direct thoughts of the character as well. So maybe it makes as much sense as anything. It's probably just stylistic variation.