I see this excuse all the time, but why not? This calculation does not need to happen more often than the product prices are adjusted. There's no difference in effort between labeling something "$5.52+tax" and labeling it "$6".
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.