Why are movies generally at least an hour and a half long?

Question:

Why are movies generally at least an hour and a half long?

I can think of dozens of films which if edited down to 20 or 30 minutes could have been excellent, but were pretty unwatchable at their release lengths.

SOME films benefit from greater length, of course. But do you know what I mean? I always think the people who make the trailers should be the ones responsible for editing the films themselves.

You only get to see "shorts" at indie festivals. Why don't big studios release them too? They could be shown in groups.

Best Answer - Chosen by Asker:

It's all about money. Market studies have shown that a lot of people get antsy when a movie runs too long. I guess the studios figure they wouldn't make enough of a profit by distributing shorts.