Why is passive immunity of much shorter time or duration than active immunity?

With active immunity, the body has cells that produce antibodies to antigens composed of amino acids. The antigen could be a protein itself, or on a virus or bacteria. Once the body has a cell that can produce antibodies to this antigen, it differentiates into memory cells and plasma cells, which are the factories that produce the antibodies.

With passive immunity, just the antibodies are given, such as an injection. Mothers also confer passive immunity to their children through breast milk. IgA (one of the 5 antibody classes) is secreted. This is temporary as it will bind the antigen, but will not induce production of their own protective antibodies.