That’s because they are competing for world first so they will do anything that will
give them an edge. The problem now (and has been for a while) is that most people think they are on the same level or have to play the same comps as them to do content.
I understand some classes just aren’t as good as others, but I think a lot of guilds/people should be looking for the player, not the class. Majority of issues in downing content are mechanics and/or people paying attention; not the extra dps/healing/etc from playing certain classes.