I'd be impressed if I managed to DM one group that managed to be heroic.
Evil campaigns are the norms from where I sit.
In my experience though, most players want to wear a white hat while having a dark soul. However, most think that they are wearing a black hat but are the good guys. They tend to mistake style for substance, and substance for style.
For example, in my campaigns...
a) The PCs are generally on better terms with evil NPCs than good ones. They show greater admiration for NPCs that are ruthless, cruel, and vindictive than those that are honorable, compassionate, or kind. This remains true regardless of the actual capabilities of the person involved. I've been casting potential patrons across the PC's path in my latest campaign for a while now. I've finally managed to get one to stick in the form of what is basically the matriarch of a mafia clan - a ruthless LE to a fault character who only wants blood and is willing to pay for it. The PCs are happier I think as hitmen than heroes.
b) The PCs believe that the outcome justifies them being as ruthless as they can be.
c) The PCs act as if killing things and taking their stuff is the goal of life. The only thing that motivates them to risk the opportunity to get stuff, is the opportunity for vengeance.
d) The PCs act as if moral behavior is important only for the sake of appearances, but that the substance of life is being cunning enough to get away with it.
e) The PCs act as if ethical constraints were things to be artfully evaded and exist solely to be given lip service and create confusion regarding your real motives. PCs was gleefully conspire to be wicked. If they must be good, they see it as punishment.
f) The players themselves are generally not conscious of the archaetypes that they are choosing and do not realize how they are seen. For example, they might not realize that in the eyes of society they are a witch, a monster, a beserker, an assassin, and the fearsome servant of a dread god of death. And to the extent that they are conscious of how they appear, they think that it's ok - they are the 'good guys' anyway. They just think that they are cool anti-heroes, when typically they act more like anti-villains.
g) If pushed by the narrative in a moral direction, they'll far more strongly resist being good than being evil. For example, if presented with two equally powerful magic items, one of which requires the player to be good and the other which requires the player to be evil, the evil one will be by far the more popular.
It's not just the one group. I love The Order of the Stick. However in one sense its not very realistic. Most groups lack anything like Roy. Belkar Bitterleaf is by far the more common sort of character. In my current campaign, I'd deliberately planned for the possibility of the party joining with 'Xykon', however, I think that this is unlikely as an option at this point precisely because Belkar Bitterleaf is in practice such a stock character. The only thing unusual about him is he subverts, to some extent at least (see Darksun) the Halfling trope.
The majority of players want to eat their cake and have it too, and are uncomfortable with the idea of an openly evil campaign. And, as a DM, I'm someone uncomfortable with the idea of an openly evil campaign, as I don't know what I'd do to out Herod Herod. It would be like needing Black Dog as a counterpart to White Wolf. When you are already a monster, the only place to go is a sort of campy over the top scene chewing evil.
I'm not sure what causes all of this, but the older the player in general the less moral restraint the PC will have. The only time you see moral questions taken seriously in the game is by young kids. I think this is because young kids know that play is a serious, sacred, and earnest affair while old people believe 'it is only a game'.
And I'm not the only one to notice that. I remember watching an episode of The Dungeon Bastard where the guest bemusedly remembered how his younger self would 'waste' wishes maturely trying to make the shared world of play better, while his older self would selfishly use them not only for his own ends, but for his own ends of dominating over others. For that matter, much of the humor of 'The Gamers: Dorkness Rising' comes from this recognition of the immaturity of grown up gamers.
In one sense, it doesn't 'matter', in that it is only a game and everyone generally has fun. But in terms of the narrative of the story, it always bugs me that any mature art that I wish to create in gaming is shattered on the obdurate wall of player immaturity and that much of the RPing I have admired over the years by a player has been by someone who was 8 or 12 or 15. Very few memorable scenes have ever come out of me playing with adults, in the sense of a scene where I as the DM can sit back and enjoy the show.