Why does your game HAVE to run at 40 however?
Consistency.
Even though objects may be moving accordingly to time elapsed, it doesn't mean that time feels constant to the player. For example: You write a game that usually runs at 60 fps but occasionally can drop to 40 fps. During the times when the framerate drops, the player can experience what appears to be slowdown even if using delta time adjustments because of the extreme change in framerates (30%). Forcing a 40 fps framerate will keep time elapsed between ALL frames constant and thereby making time FEEL more constant.