"in the chair" and "out of the chair"

Why do we say "in the chair" and "out of the chair"? Shouldn't that be "on the chair" and "off the chair", as there's no inside to a chair, so it's not possible for one to be in it.

If it's an arm-chair, it's kind of insidish.

Actually, you can say "off the chair", which is synonymous with "out of the chair". Similarly, "on the chair" is also valid, but it is *not* synonymous with "in the chair"; rather, "on the chair" means that someone is standing on top of a chair or something has been placed upon a chair. (You generally do not say that some inanimate object is "in the chair".)