People move, die, get married/legally change their names
All things that are already registered in the government's basic citizen administration (or whatever it's called in your country). Deaths and moving to a new address are both things you are required to register with the government ASAP. Marriage and legal name changes are performed by the government (it's kind of the entire point).
Computer glitches and human error occur. Bureaucracies aren’t designed for efficiency.
No, but a person's whereabouts and legal status are necessary for taxes, and if there's something governments are very good at it's collecting taxes.