because primers can’t be punched out and destroyed and firing pins can’t be swapped out or modified by even the dumbest criminals.
As I recall, it was claimed that the microstamping wears out after a hundred rounds - especially if the owner makes no effort to keep the primer area free of fine dust and sand.
But, of course, the purpose of the law is not traceability of the brass. It's to further complicate ownership of firearms, and to limit their legal supply. Illegal supplies are not affected - one can drive an 18-wheeler full of firearms across the CA state line.
Additionally, anyone can come to a range and collect as much empty brass as he cares, with all the microstamping, and leave it at the crime scene. The police will happily execute a 3am arrest, in best traditions of Stalin's NKVD, on an entirely innocent person - this will buy the real criminal an extra week.
Probably be against the law to change the firing pin.