There is more to a racing sim than just tagging something with "simulation mode." You need accurate models of the cars, tires, weather, tracks, track dynamics, physics, etc. You can't take what you learn from GT and Forza, and expect it to work in the real world. There are too many "assists" and unrealistic models in those two games that it still isn't up to par with the simulations. Most of the driving mechanics from GTR, iRacing, rFactor you can.
As stated before, a lot of pro drivers train and practice on iRacing, and many team/manufacturer simulators base their engine off of iRacing. iRacing was based off of the Papyrus games, which back in the day, were pretty realistic for what was available at the time. It has only improved since then.