You don't have to have a tank to win. The problem is people don't know how to play with the comps they pick.
Enjoy your leaver
When you're the ONLY tank on your team, and the enemy team is four tanks:
https://s-media-cache-ak0.pinimg.com/736x/d7/92/ac/d792acbf52b5e5618c1ac4cd615e386c.jpg
I've won plenty of games without a tank. But those were games that ended really quickly, usually ending before 25 minutes due to pushing huge leads early on.
But if a game reaches the 25-30 minute mark and you don't have a tank, well your screwed.