But there are millions of kart racers out there, and many of them are pretty poor. STK is better than most of them.
Like the reason "the year of the Linux Desktop" hasn't happened is not explained by pointing out that there are a lot of OSs and Linux is better than most. It hasn't happened because everyone is using Windows and MacOs. If STK isn't comparable to Mario Kart, then it isn't good. Everyone is playing Mario Kart; that's the standard to beat.