Rfactor was made for modding so judging it on its original content is a bit silly really. If you ever buy the game (i strongly suggest doing so) get the R1RFT mod and the Historic GT & Touring car mod. The physics on both of these are as good as LFS and the massive selection of tracks you can drive them on means that Rfactor well deserves its spot in the most influential sims.
Another quick though - influential racing sims is the title. Not best racing sims, Rfactor has influenced the modding community so greatly I guess it would be silly not to include it, I guess.