Hasn't the franchise always been 60fps?
Forza 1 was 30, but ever since Forza 2 silky smooth 60FPS has been very important for the series. Forza 3 and 4 had very little smoke because in 2 player races this could cause a frame rate drop, that's how important 60FPS is for them
Fun fact, the rendering is 60FPS, but the physics are actually running much faster at 360hz in 2, 3 and 4.