I guess it's because everyone would use RS then, and wouldn't get to experience much with any other tyre. SH is my preferred tyre, on any car even flat out race cars, SS are good too. These sport tyres actually give a proper driving experience, a sense of speed, acceleration, power and handling. They also don't spin out insanely if you clip the grass by a millimetre.
I'm glad career forces tyre restrictions as it's basically the only time I get to use them, online and it's 99.9% RS unrestricted. A lot of people really seem to under appreciate the driving in the game, RS are good for tearing about but they feel so arcadey, insanely so on 500PP and below.