The store is uniform across geography, in the sense that it doesn't move and a system within the store is capable of calculating the taxes. If there was a simple flat "10% sales tax" on every single item in a store, the store could simply print price tags with that included. Marketing materials posted in the store could have the advertised price slapped on a sticker by the store too.

The issue is really that taxes in most places are not uniform in application, not necessarily just because of geography, but also all of the specific carve outs and exceptions and so on that America tends to have, e.g., SNAP and WIC often don't pay sales tax. So its actually hard to know what someone is going to pay without communicating with them personally beforehand.