Simply, it isn’t very fun to get Ghosts/Gungooses/plain Warthogs/Choppers/Banshees/Wasps as drops. They’re not strong enough or tough enough to be worth the fight most of the time and it just pollutes the pool of potential drops for vehicles.
The vehicles the Pelican drops off should be major, game-affecting vehicles like the Scorpion, Wraith, Rocket Hog, Gauss Hog, something like the Sparrowhawk or Locust etc. if they get added later. Light vehicles like the Ghost/Gungoose/Chopper should all have at least one permanent spawn point in each base on each map that rotate per game like weapons do.
If a map has Wasps/Banshees, then they have a spawn pad that works through the whole game too.
Heavy vehicles should also drop at least 2x a match, or if a match is stale mating and it needs to be shaken up a bit.