Why is 4 the magic number?

PIC UNRELATED BUT AWESOME NONETHELESS!

Let's face it, splitscreen multiplayer is sadly going the way of the dinosaur.

Gear of War 2 opted it out of online multiplayer (edit: If you have two gold accounts you can splitscreen online with Gears 2 so they opted guest online splitscreen out) while the only other game with multiplayer that supports the feature with several players is Haze and good god if your being bested by Haze in anything go drown in a knee-deep lake you failure. What I don't understand is why, if splitscreen multiplayer is apparently a dying breed, developers carry its same restrictions into their online co-op games.

4; So far most co-op games seem to put this as a benchmark for what they hope to achieve with their cooperative modes. Crackdown 2, Left 4 Dead and L4D2, Halo 3 and ODST, Borderlands, Haze, Call of Duty: WaW, Lost Planet 2, Rainbow Six Vegas and R6V2 all mostly new games (some not even having seen a release yet) that go beyond the 2 player co-op norm with 4 players. Now of course 4 is better than two but seriously why can't we have 5 or 6 players? You could use the excuse that it'd be too difficult to handle that many players at one time with the AI and scripted events all going on at once but if MAG can get 256 players in a competitive multiplayer match and still look decent I'm sure devs can manage. Hell I know they can manage because Insomniac and Epic games have, they've introduced separate co-op modes, in Gears 2 and Resistance 2, that allow beyond 4 players with Insomniac actually fitting in 8 if I'm not mistaken. In the few games that 4-player local co-op was an option, again Haze, it was set that small because a television screen couldn't ft more than that, or if you consider a 360 or Wii game the console's inability to support more than 4 controllers at once, but the beauty of online multiplayer is the treat of your full television screen and need of only one controller slot.

I know that was pretty much a incomprehensible rant but it makes no sense as to why more devs aren't taking advantage of the new online gaming boom that's happened this console generation in terms of furthering the co-op experience. Even 4-player local multiplayer games don't exactly get it right. Why can't Online Castle Crashers allow for 5+ players? Why does Marvel Ultimate Alliance 2 sit at 4 players? Why does the one 'Warriors' game that allows for several players in its co-op campaign decide on 4 people (Gundam Warriors blah no one cares blah for PSP)?

Maybe I'm just a whiny bitch but has it never occurred to someone else that with the technology to talk to a goddamn skynet super AI that maybe we could fit in one more person in co-op sessions?