Zip isn't uniformly distributed numbers though so you dont have the equivalent of that many digits of decimal numbers. Other comments have more detail but just for the top level example the first number is the zone and goes from 0 on the east coast to 9 on the west.

Only if there's never more than 10,000 addresses in a single zip code, which means that if you enforce that, you can force a zip code to appear by building enough house

That happens, and worse. I've lived in multiple areas that have had their zip code changed. Area codes, too, sometime more than once.

I think my point is that if you're going to make people learn a 9-digit identifier for their house you might as well make that identifier unique and then that's the only information they need to fill in. Having non-unique 9-digit identifiers feels wasteful.

There are far more addresses than there are houses