Probably because DHs aren't godmode; I lead a Mythic raiding guild and our 2 DHs are only high on AoE encounters – their single target is decidedly middle-of-the-pack, and even their AoE is comparable with other strong AoE specs. If you're struggling to beat them, you're either looking at AE damage as a class that doesn't excel at that aspect of the game, or you're being outgeared/outplayed. In a situation where everyone has a similar ilvl and skill level, they're not ridiculously strong.