I agree it's difficult to switch between sims if you don't have much time to spend into them, i pretty much only play pCars2 for casual online (no time for leagues), RF2/PC2 for offline racing and RBR for rally. I have all the sims but iRacing, but the only one that has some interesting content that brings me back to it sometimes is R3E. AC, ACC and automobilista are not even more on my HDD, the first two because the phisics/FFB are different enough to need to re-learn them every time, the latter because of no VR support.
But then, i think it's also about publishers going too much all into the same direction, there are LOTS of interesting racing categories, yet no sim offers complete grids for Group C, Can-AM, supertouring, JSGT, '00 DTM, etc.
I'd like to see some free-roaming sim, something wreckfest-like, an hillclimb sim, there are LOTS of unrepresented forms of racing, i know GT track racing is obviously liked by fans, but it's also obvious that nobody really "needs" more sims having it, pCars has like 14 GT cars, rFactor 5+mods, ACC only them, etc. I think we have enough choice even if you don't like some company/sim.