Obviously there are significanly more zip codes than states,there are also much more factors to eal with than that. To tac this properly there would have to be a registry of all potential addresses I the nation, special rules for PO boxes etc.
For example, I live in a small municipality that has an additional tax rate, surrounded by mostly unincorporated portions of the county. However, we are small enough to not have our own zip code. How would I be taxed? Tax everyone in a zip code at the highest rate? The lowest?
This situation is not unique, and oversimplification of the rules means it's still not "fair" for everyone.