Why do they play so loud?

I often go to concerts, and too often I find the need to use ear plugs because of loud sound levels. I really don’t get it, why is it necessary to play so loud all the time? Usually lots of people around me agrees that the music is unpleasantly loud, and I often see other people using ear plugs.

I have bought expensive ear plugs a couple of times, but I always tended to forget them (eventually loosing them…), so now I have just bought lots of really cheap ones so that I can have a pair in every pocket. I don’t like listening to music through bad earplugs, but I work with sound and really have to try and protect my hearing.