Isn't yoga the most spiritual form of exercise?
This is sort of like asking, "Isn't Exxon the most beneficial brand of gasoline for human beings?" Gasoline is fuel for automobiles. Humans require entirely different fuel in order to be happy. "Spiritual" refers to the eternal self within the body. The material body needs exercise. The self can exist most naturally and happily in the spiritual world, without a material body at all.
As a form of exercise, some kinds of yoga can be very beneficial, if practiced correctly under able guidance. The same could be said for any physical fitness program. The sitting postures—asanas—practiced as "yoga" today are only the beginning stages of a yoga system from a bygone era, when people practiced for years in seclusion as a means of self-realization. But that doesn't make contemporary yoga "spiritual" in and of itself.