Because they don't want the other 5 included in other bonuses which "double", etc, your earnings. (And they want to be able to call those other bonuses simple terms like "double" instead of complex phrases like "double if you get 1 or 3, but one-and-a-half-times-normal if you get 10".)

And it's not unlike airlines and hotels, which separate the concept of "base" miles or points (which get doubled or tripled in "double miles" or "triple points" bonuses) and elite bonuses (which don't get increased by other bonuses).