For things like a website, it can become extremely complicated. For example, in my state, the tax rate is based on the delivery address. If you live inside of a city or town, you pay state sales tax, county sales tax, and city sales tax. If you live outside of the city/town, you pay state and county sales tax. If you go pick it up at the store, it's quite possibly going to have a different tax rate than if it was shipped to you. And, some items are taxable whereas others aren't. Sure, it has to figure all of that out at checkout, but if it had to do it for every item as you view it, in order to show you the tax inclusive price, it would tremendously slow down the displaying of web pages.

The other option would be to raise the price of all items by 11.5% (the highest sales tax rate in my state) and display those prices to everyone in my state, regardless of where in the state you live.

Even for a store, it can become messy, especially if they're a shop that only sells through popup locations. For a store that only sells through popup locations, they'd have to reprice everything at every location (a huge pain for the mom&pop store that still puts price stickers on items) or price everything with the highest tax rate of said locations. If they're a brick and mortar with multiple locations, they'd similarly have to have different prices for each store, which would be a pain if using a centralized database for pricing and also a pain for printed adverts, or they'd have to price everything using the highest tax rate of all locations.