because those amazing missions
https://snag.gy/gjnpwt.jpg
divided by 4 will simply make you never play any wing missions again. Regular trading itself exceeds the payout of those missions even without division by 4 or 2. So whats even the point of these 3 rolled missions? Some kind of intelligence check to see who has the economical comprehension of a space ameba?
What you have pointed out there is a balance problem, not a good argument in favour of multiplying payouts based on the numbers of players. If the rewards were doubled for those missions but then distributed amongst the wingmen, solo players would earn double the money (which would be about 1200 credits per tonne, which is pretty reasonable), 2-ship wings would be completely unaffected while 4-ship wings would no longer get brokenly large incomes simply by virtue of having friends.
Plus you are only neutral with that faction, obviously they aren't going to give you decent missions. Most of the wing missions I've seen are 3000+ credits per tonne for a single hop, which beats out all but the greatest commodity runs on top of being much easier to find.