Because god forbid DPS get held to very fair and reasonable standards eh?
If you genuinely feel that expecting DPS jobs to do more DPS than a Tank in an encounter that's pretty much an ~eight minute target dummy is in anyway elitist or even just unfair then frankly I have no words for you.
At the end of the day, this is a team game and in proper raid content I'd like to think that players are obligated to at least put a baseline amount of effort into performing at a reasonable standard.