Well for starters Race 07 and GTR Evolution can barley be called realistic.
Not everyone has the time for (or even the means of getting to) a track facility, and the level of data iRacing uses means that it is the closest virtual thing. People have been spending hundreds if not thousands on wheels, cockpits, pedals and such to try and get the best sim racing experience for years, so when a sim comes out with an unparalleled level of realism and investment, lots of people won't mind coughing up the yearly $156 for it.
But if you can spend less and enjoy some real life racing, then that's fortunate for you.
EDIT: My nearest kart rental is over an hour away, it costs nearly £1 a minute, I don't have a car and unless I go with another 10 or so people, its not actually a race.