Street cars typically have a stiffer rear rate for the simple reason that they try to account for various situations where more weight may be added such as additional passengers or cargo. It's tough to try to get an optimal setup for all situations when variables can keep changing. In other cases a manufacturer may just use what they already have for price reasons. Weight in fuel is also constantly changing. I wouldn't use a street passenger car as a basis for comparison. Real race cars may be limited in geometry due to a design envelop specified by the rules which means unconventional solutions to deal with them. Current F1 cars are a prime example.