Why are almost all anime episodes 20 minutes long? Why are all animes around 20 minutes long? Sometimes less or more due to the opening and ending, and some recap from last episodes...etc, but it's always around 20 minutes.

Why is that?
Presumably it's to fit into the standard 30 minute time slot allocated for the show.


20 min. episode
3 min. OP + ED (90 seconds each)
Commercials


And it's about 30 minutes.