Why does rock concert sound suck?

WAY too loud.  That is my major complaint.  I could deal with the distortion if it was at 90 db or less.  Plus, if was less loud the distortion would drop considerably.   We see concerts that are not "rock" and big venues and they are amazing and very clear.  

I don't go to rock concerts for sound quality it just needs to be loud. And I've found MSG to be a good sounding venue.

Been to plenty of rock concerts in my years. I no longer attend many of them, as they're way overpriced these days (for my budget), and yeah, IMO, with all of the latest fancy sound gear that's out these days, I'll chime in also saying that I think 95% of the time the sound is not to my liking. Summerfest in Milwaukee WI comes to mind. Plenty of outdoor stages where you can walk from one to the other. Almost all of the time the subs are overly emphasized, and the rest of the mix is muddy. I've thought this numerous times when being there. I happened to see Nick Lowe in the past few years at another location, and my friend and I both thought the sound was horrible, so much so, we didn't even stick around for the whole show. I happened to be at a State Fair this summer and heard a stage where a 12 piece big band was playing along with dancers and singers (lots of wireless mics for the singers dancing around). There was probably a crowd of 1,000 that was there for this performance. The sound was absolutely perfect and amazing. Spectral balance and loudness level was just right (IMO). Sometimes, for my taste, some get it right, although these days, I'm most times much happier simply enjoying my system at home.  

Most venues are multi- purpose. I find older venues sound worse. Heavy metal is difficult to mix, Saw Iron Maiden on their last tour and sound was great even with a loud crowd. Saw Judas priest and it was not near as good. After touring 40 to 50 years Iron Maiden has it perfected.

I have to disagree with the central premise of this thread, and kindly suggest that some bands should not play some venues.  I saw My Morning Jacket this summer at a small outdoor theater in Dillon CO.  The sound was impressive beyond belief.  I do station myself as close to the mix board as possible, and in this case was directly to the right of it.  I also saw Spiritualized recently at a small indoor venue in Chicago and the sound there again was very good.  GA show, make sure you're not under any overhangs or balconies for an indoor show and you're in good shape.  

If we are talking repurposed basketball arenas with assigned seats then yeah, not going to be the best experience.  This technology though is worth keeping an eye on....I thought U2 dabbled in it prior.  

Last thing I'll add is some people want the live experience to sound exactly like the record.  F that.....to me the live experience is about how well the band adapts, evolves and presents that song in this new venue outside the studio....that's the true test.