Well... why do we?
If we need it to cut the colonisation hauling grind, the problem you face is that the same guys releasing new ships for you are the guys balancing the colonisation hauling grind. They want it to take a certain amount of trips. Yeah, they could give you a single incredibly expensive ship type that can haul 5000 tons of cargo... or they could cut the colonisation grind by 80%, and you get the same benefit in a way that doesn't completely distort the ship market. But if they instead don't want to make colonisation vastly easier, they just won't, even if they do give you a 5000t hauler. They can just inflate those gains away by increasing the grind.
If we need it because that's the deserved legacy of the Panther-Clipper, then I don't agree either. Yeah, it carried a lot, but it wasn't even top dog in payload (that was the Grifin), and its payload was only 18% higher than Frontier's version of the Imperial Cutter (the Imperial Explorer). Also, I think sometimes people can get into the habit of looking on wiki.alioth.net, seeing the Panther-Clipper had a payload of 2100t and translating that directly into cargo capacity in this game. It's not comparable, not only because it's a different game with different balance, but because a Panther-Clipper in Frontier didn't carry 2100t cargo. The payload had to include the hyperdrive, after which the cargo was reduced to more like 1200.