Sales tax can can vary based on the consumer (e.g., purchasing on behalf of self vs. on behalf of a non-profit), and regions change sales tax seemingly at whim for stuff like paying for a new local stadium. There are more than 10,000 different little sales tax regions in the United States.

Easier to compute at the register than redo a zillion price stickers.

It’s also just the way it’s done. There’s inefficient things in every country in the world. It’s just how it is and it would take too much of an effort to change things for countless shops and stores.