That's cuz either the pilots on a lot of Chema or they get shot down by the odd missile launcher... If they travelled by land then they'd most likely come across a lot more problems - it's also a lot easier to take things down from the air - remember the first mission you're given from elder maxwell on the prydwen? Unless you tell the vertibird to land so you can fire mini nukes, you can take out the behemoth and any other enemies quite easily with the minigun hat has infinite ammo