Because sales taxes are ussually set by the local jurisdiction.
So it could 10.25% in one place, 9.75% in the next town 8.750% and 8.975% if you drive for another 15-20 minutes. Imagine if you run a business, have multiple locations and want to buy a TV/Billboard/etc. ad with the price how would that work? You'd also need to make different price stickers, won't be able to print the price on the packaging etc.