They use plastic because its cheap, easy to use, and lightweight. That said, even some of the metal stuff breaks, too. I can't tell you how many metal buckles I've broken on duffel bags. Btw, the Army supplies replacement plastic buckles with the latest issue of ACU colored field gear.
Funny you brought up Everest, though. A plastic buckle broke on my friend's pack during our trip to base camp last summer. It wasn't a very important clip. and she didn't do anything crazy to it, it just broke in normal operation. I've had plastic clips break during normal use, and not so normal use such as getting run over by a truck, dropped, and stepped on.
When I go diving, I like to dive, not fix things.