There is a huge range of skill levels in the lobby so it's often impossible to balance teams perfectly. Even if the player skills are balanced, civs and counter units screw it up again. Having broken champ cavs doesn't help and civs not having basic counter units make it worse.
Furthermore, you get smurfs misreporting their level or noob players claiming to be pro just to be allowed into a game. And then there is Geriatrix trolling you with new accounts every day, in which he plays badly on purpose or throws tantrum and ruins the game for everyone.
It's also possible for a theoretically stronger team to lose badly because one guy throws the game or if someone ragequits.