I certainly haven't even seen D&D being like that, so I agree with you here.
And not like there's a shortage of RPGs which are like that compare them to. The vast majority of White Wolf RPGs (including the non-WoD lines) were absolutely aiming for "Watchmen not Avengers", whether they're Vampire, Exalted, or Aberrant.
D&D has, since 2E at least, always been aiming at a different, younger and less grim-oriented market. As you say, there have been the odd book that's a bit "edgy", but y'know, compare Book of Vile Darkness to, say Clanbook: Tzmische (sp?) and BoVD looks like pretty schoolboy stuff.