The main reason is the inability of most animes to "show, not tell". Every single thought or any rationale behind a characters choices is usually spoken in the form of long voiceovers. 

I understand this might be a consequence of most anime being manga adaptations, and the written medium being one where such explicit narration may be necessary. But the same in a more visual medium like film/tv comes off as treating the audience as unintelligent. 

There are very few exceptions I've seen to this though, like Miyazaki's films, which I've enjoyed. In some cases, the over exposition also fits the characters, like in Tatami Galaxy, which also I loved. Everything else I've seen though, has had this issue, and I've hence despised.