The issue with that is that ZIP codes don’t map physical locations, they map the hierarchy of how the mail system does routing down to each post office and were introduced in the 1960s [0].

As a result, doing something “from the start” wouldn’t involve baking in comparability with the quirks of a piece of software written decades later, and you’d also have issues with, for example, single zip codes spanning multiple states.

[0]: https://en.m.wikipedia.org/wiki/ZIP_Code