The reason I think that others haven't tried it is because they are not nearly daring enough. I think that they believe that each game is divided into each genre, eg. FPS, RTS, Sport etc. etc. They might want to combine genres but think that it may not have a good effect; that the adverse affect is they deter supporters of each genre because they like one and not the other.
There is a wealth of old topics with useful and unique information - your question may already have been answered!