It's just one of those things in English; sometimes if you combine two words you get a phrase that has a different meaning than each word by itself. When I think of another example, I'll edit my post.
I would note that as to is okay to use, but it less preferable than concerning or regarding. (One of my coworkers from Romania likes to use "related to" — as in "Related to the initialization code, we need to add something to detect errors" — and it's perfectly understandable, but not natural-sounding, and I haven't got the heart to tell him this.)