That's because it is a military aircraft. It started out as a civil cargo concept, then turned into a military application.
Anywho, no need to nerf the 757F (which would also nerf every other Lg aircraft). The 757F works great in MH, smaller markets. US/EU are the perfect playgrounds for it. However, once you start going beyond 5000nm/100k+ kg cargo routes, the 757F is really not that OP.
1. Distance makes no difference
2. As long as the system allocates equal amount of cargo (100% of revenue) to an aircraft that is 33% of cost of Very Large aircraft, I am really curious what your definition of what Over Powered means.

300% efficiency is not overpowered enough?
3. FedEx is flying Very Large aircraft just fine over MH, without having to worry about someone showing up with a Large aircraft and emptying their Very large aircraft
To counter that strategy, you just have to fly pax as well to level the frequency benefit out. I'm fairly sure the system doesn't care if the flight is cargo only or not. Each frequency is counted separately so if your going against 2x daily tech stopping 757F, then a 1x daily cargo and 1x daily pax/cargo (combi/pax+belly) will even that out.
You cannot combat it if AWS artificially created 200%-300% efficiency of 757 vs Very Large Aircraft.
Cargo is new, and very few people have discovered, and have employed this exploit, does not mean that its not imminently coming.
I am curious what happens when we have double tech stopped A321s flying with 200% profit margins of single tech stopped 757F, 600% profit margin of 777F.
Because the system only recognizes
(arbitrarily) the double tech stopped A321 as
1 flight, single tech stopped 757 as
1 flight, non-stop 777F as
1 flight.