Sales tax varies by state/county/city. It is generally not cost-effective to have each individual store label all their products with local sales taxes applied.
Most of this could be resolved by not putting the prices on the products themselves, but that isn't as good of an experience for the shopper.
That is already often the case. Prices are usually on the shelves not on the product itself at many stores. And when purchasing online there is no reason that the sales tax couldn't be included in the listed price.
Also sellers could just charge the same price everywhere and take the sales tax out of the revenue.