Grammar frequently doesn’t make sense. When you find yourself asking why, the answer is usually “that’s the way it has evolved.”
Macmillan Dictionary Blog has taken on the question of the logic of grammar and expressions in one of their recent posts Could you care less?
“We make the rules; they don’t spring from some utopian realm of transcendent consistency.”
What I love about this post is that Macmillan argues that expressions don’t have to make sense. “I could care less,” one of those expressions that doesn’t make sense at all, actually means “I couldn’t care less” (read: I don’t care at all).
Why? Because everyone started using it that way.