Why aren't there any? Save for Regicide and, I suppose, you could count Flood. But those are two very specific gametypes. Why no Rumble pit with a pool of FFA- King of The Hill, Slayer, Oddball, Juggernaut, etc.?
And what about Head To Head? The most suspenseful and arguably exciting gametype ever to grace the game.
It's hell trying to play any sort of objective-based game without an organized team because, as you are all aware, no one wants to work as a team with a group of strangers they were just matched up with. It's usually one of two mentalities: A) I just want to run around and kill people so I've no need to work as a member of a team or, These guys are all fools. They're beneath me. I needn't deign to work with such lowly creatures.
Sometimes, you can't gather enough friends to field a team. Sometimes, none of your friends are good enough or interested in the gametype you want to play. And, for some, team-based games are just plain dull. Some people are 'lone wolves' and don't like playing with a team period. Are we to leave them out?
This is the reason for FFA and 1v1 playlists.