I guess they figure if you're will to pay for a 1/5 scale car, you're willing to pay 1/5 the price for tires. ?
The cost of rc tires are pretty outrageous. there isn't much engineering in them, compared to a regular car tire. I can buy a tire for bicycle for the same price, and it requires more engineering and a heck of a lot more material as an RC tire.
The reason why? They will sell a million bike tires and only sell ten-thousand RC tire, as an example. It's all about selling in quantities.