At the round start, people buy a lot of vehicles, especially jets. After the initial fights, this can lead to team having 3 or even more jets while the other 2 teams have none.
Buying a jet in this situation will most likely get you killed, although you might take one of them with you. In other words, you waste 10k+ for 2k. No one will go for something like this unless he has insane amounts of money.
I would like to see a feature that, once you repaired your vehicle, similar to how you get teambalanced when respawning, forces a sale of the vehicle you are in, for the sake of balance.
While I know it is frustrating, this is probably the only way of getting rid of stacked vehicles, especially jets, since they are basically untouchable with ground AA unless they do a stupid mistake.
And if the players decide to do nothing (not engaging because he is afraid of loosing flares and not rearming because he does not want to loose the jet), you still took their ability to use the stack to their advantage.