Zipcodes are not great for reliably describing locations. They're collections of multiple postal delivery routes, and that's it. There's no guarantee that any given zipcode lies within a single city, or a single state, or that it lies within the boundaries of the US at all, or that it's contiguous, or that it's stationary (there are some for boats), or that any given location only has one, etc., etc. People think this is a good idea because their particular case happens to work well, but zip codes are rife with edge cases.