You're matched with people of a similar level. So right now, you're playing with 24's on, assuming you're doing Tigers, a level 24 strike. Once you reach higher levels, people won't quit on you nearly as much, for two reasons. One, they'll be more experienced and hate people who leave just as much as you do. Two, the level advantages will make the already fairly simple strike playlist much, much, easier, so there's no reason for them to quit. The worst you'll have to deal with are AFKers, not so much quitters.