Why?

I have a 4 digit postcode, I have to look it up every time I have to fill in an address form for delivery.

I've had people screw 1 digit up in that postcode and their items (a laptop in one case) went to the completely wrong city.

A code sounds foolproof, until you realise most people don't engage with them for most of their lives - you don't tell the uber driver the zip/post code you are waiting in, and travelling to, nobody does.

edit: just to add - Magic numbers are bad. Software engineers know that a number that's undocumented in code is unmaintainable, a zip code is worse.