It's a legacy from the NiCad/NiMh days. Cars were designed to work with a lot more weight than they have today, so the easy way to get things to work is to add weight back to the old levels so the designs we have work.
It's about getting enough weight on the tyres so they function effectively. Other classes have effectively dropped their weight limits to make the most use of the LiPo technology, but they have softer tyres to work with. It would be interesting to see how well the cars worked with tyre compounds that were a lot softer. However, I suspect that the cars wouldn't fly well or give any reasonable traction despite the softer tyres.
It isn't poor or lazy design by the manufacturers, it is that none of them have managed to find a design that works when you take a chunk of weight out of the car. Seems like we got it right all those years ago...
