Plus wouldn't they be rare seeing as you have to manufacture them and their components in a dedicated aircraft manufacturing plant.
You can't just put them together from metal frames welded together in a workshop and put components from salvaged cars and perhaps the one or two planes you come across.
The Enclave had access to pre war standards manufacturing facilities they probably have been upgrading since the War, the BOS on the East Coast had to do with what they could salvage.
Even their own Power Armor is what they found in the armories of destroyed military bases.