Because it's against the law! Is that so difficult to understand???
If you do not own the copyright (i.e. "the right to copy"), you do not have the right to copy. It's a federal law. Simple as that! Do you still need to ask "why can't I?"
For your convenience, the relevant parts of the above link state:
the owner of copyright under this title has the exclusive rights to do and to authorize any of the following:You are not that "owner of copyright", so you are barred from doing either of these. Pretty cut and dried.(1) to reproduce the copyrighted work in copies or phonorecords;
(3) to distribute copies or phonorecords of the copyrighted work to the public by sale or other transfer of ownership, or by rental, lease, or lending;