Why can't Shor's algorithm be run on a classical computer?
Shor's algorithm requires the use of a quantum computer, because it relies on the properties of quantum mechanics. Specifically, it uses the fact that a quantum computer can perform operations on many different inputs simultaneously, in a process known as superposition. While it could be run on a classical computer any increases in efficiency it offers over traditional factorization algorithms would be lost.
Do you know any open-source libraries which allow running Shor's algorithm on usual PC?