Transports are too big and clumsy for landing on spaceports. Why would you do such a thing, flying that massive train between skyscrapers? That's silly.
Now transports can't dock on a planet. What do you do? You bring along/hire a freighter to offload for you!
That's right, teamwork baby. Helps keeps things moving.
So make trading harder, right? Screw those guys and their 5kers.
Yes, but no. Give a massive bonus to the profit of those items on planets. Maybe 50-100% for all of the already high-priced commodities, and maybe even up to 300% for generic commodities (food, water, etc). The numbers are just an example, don't get mad now.
What does this do though?
Freighters now have a reason to exist, and while transports can still dock on a station somewhere nearby for profit, the real cash is on planets. It encourages convoys, teamwork, and gives freighters something to do if they just happen to be nearby.