Because it will be game breaking for the pilots that don't like big ships, and like being a pirate.
Fighting 2 of those fighters, AND the mothership, is impossible. It's gamebreaking impossible for that particular player.
With only one fighter you as a pirate have a good chance on succes regarding the decisions you make, and the T9 pilot you are about to pirate, also does have a change to win regarding the choices he makes.
Its balanced this way.
and decreasing the DPS of those fighters is not fair for the pilots that can only carry ONE fighter instead of 2,3,4,5 ect.
And, NPC's will be deploying fighters as well..
I dont want to fight 1 elite Gunship, and 2 elite fighters the same time..
You can adjust the DPS of the fighters on a scaling level. The more fighters deployed at once, the greater the nerf to their DPS, as each fighter dies the DPS is increased on all the other remaining fighters until one fighter remains and its DPS is back to 100%.
As far as I am concerned though, if someone wants to take on a ship that's 2-3 times their size, they should be prepared for the potential of that ship having multiple fighters to defend it and they would need to plan accordingly; such as bringing a wing of friends.
More than 1 fighter at a time:
a) more awesome battles,
b) more tactical awareness required,
c) more thoughtful decisions required,
d) potential for increased multiplayer experiences,
e) greater role requirements for when player-crews is released,
f) more awesome battles.
Edit: If it's really needed, then just limit it to 3 fighters - meaning a player-pirate would be taking on a wing.