NFS hit 100M in 2009, have had 23 releases on multiple platforms since 1994, compared to 9 releases for GT at 70M, and neither of those numbers is counting GT6, since 1998.
There's a ton of choices for racing sims on PC so unless GT did something better than the current crop of PC sims, I don't see them making a dent in the PC side of racing games. The strong point is being console exclusive.
Actually there aren't many good racing games on PC. Lots of arcade and other BS. Some new simulators appeared in the recent years, but that's about it. GT has substantial advantages: car detail level and amount of cars. As well as track detail level and simulation-ish physics (except some bugs). No other PC racing game maker could afford this asset.