It would be unenforceable. Prove that the person who dumps a pet was the owner (they'll deny it), and mandatory chipping would allow government to hire more bureaucrats to do what?; go door-to-door on house searches demanding to see peoples' "pet papers"? City councils are happy to spend their time writing ordinances and imposing fees and fines which cost all local taxpayers money to pay people to write ordinances and set up new government "enforcement" offices, but never make an huge impact on the initial problem.