Quoting InfiniteVengeance,
It's easy, host your own game.
No one *joins* custom games as a team, if they wanted to be a team they'd host their own game.
Thus everyone who joins your custom game is completely random.
That's not actually true. There have been several instances where people join my games and ask to hold spots for a few friends they have coming.
Also, after playing for a while, people get to know who the better players are out of a given group that they play with regularly, and when they are all on one team, it's easy to tell when it gets to be one-sided. Most people will just leave knowing the odds are against them. Having a random teams option would afford a better chance of having the teams more evenly distributed in such an instance.
Another point is that this would also make it impossible to choose your demigod based on what the other team chooses for their demigods, which would always make it a guessing game as to who you'll be against, and I find that to be more appealing than knowing beforehand. I could also see how some people wouldn't like this because they would want to choose their DG based on the other DGs on their teams (maybe to avoid 4 QoTs or 4 Sednas or something), but again, if you're worried about that, then you should find a host who isn't hosting a random teams game.