There are many reasons rock shows generally can sound awful. For one thing, and the most important one is that most of these arenas are not designed for sound quality at all. They are echoey, sound smearing, godawful places of zero acoustics. All the DSP in the world isn't going to tame that mess.
In such venues, the best that can be done is to make the show an "experience" with tons of body shaking low bass and a light show to blow your mind.
When you go to a show you aren't paying to see someone in a recording studio. If you get lucky and your band is playing a quality hall, say where a symphony orchestra plays, or a smaller theater with some sound treatment on the walls, you are lucky.
I've seen countless shows over the last 45 years, and the sound quality has run the gambit from great to crap.
My favorite venues are either outdoor ones that don't have echoey sound or smaller indoor venues having 200 to 2000 people. Think bar shows up to modest venues. They seem to offer your best chance of hearing good sound.
For instance, over the last 30 years I've gone to an outdoor music festival of Americana/Rock/Jazz/Bluegrass called Merlefest. And the sound there is GREAT, mainly because those running the soundboards actually care.
Why does rock concert sound suck?
I have been to two rock concert in the past year : Brit Floyd in Bridgeport CT and Eric Clapton at Madison Square Garden, NYC (last Monday)
For Brit Floyd I was about 40 feet form the stage and treble end was an ear-splitting distorted sound - the soprano solo on Dark Side of the Moon sounded like a chain saw running at 5x speed.
For Eric Clapton I was sitting at floor level about 20 rows behind the mixing desk - i.e., the opposite end from the stage. In this case the high top end was not so distorted, but the voices were still very harsh - seemingly a massive response peak at ~1500hz. Imagine AM radio with the treble turned up 20db.
I knew a lot of the words form the songs ahead of time of course, and just about recognized them, but otherwise the lyrics were unintelligible. The only exceptions were when he sang a quieter song - e.g., “Tears in Heaven” . Clapton moved back from the mic rather than place his mouth right next to it. Then the sound was quite listenable .
Of course managing the acoustics in such a big venue is no doubt a challenge — but does it have to be this bad?