It's a matter of volume. They scan a number in when they accept a package, and should scan it when they've made actual delivery, but tracking implies scanning it at every intermediate point from mailer to recipient, and that would slow down the works a lot.
I think the New York Post Office still handles something like the 10,000,000 pieces of mail a day that they did when I worked there.
Registered and Express Mail get noted at every turn-over, but ordinary mail, even with special services, does not.
I recently got a FedEx ground delivery that was scanned in, noted moved to their dispatch point, and then never updated until day of delivery a week later. But they did have that delivery day estimated correctly from the start, which was all I needed.