Most VAT based countries have a single VAT rate for the whole country (or possible a country-wide set of rates for different types of products). The United States is not like that. Even if we consider an individual state as the equivalent to Country, each state does not have one consistent sales tax rate.
Typically the state will have one rate, the county can have its own rate, the city its own rate, and potentially even special districts with their own sales tax rates.
That complicates things a lot. For example, if a law required the shown prices to include tax, it would mean that an online website cannot show you the price until you have entered your exact street address. Most people don't want to tell the website where they live just to find out a price that may not be any good.
The variable tax rate means that if the law applied to advertised prices shown on say TV, it would not be possible to have nationwide advertisements that show a price, or even statewide advertisements, or in some cases not even city-wide advertisements. (Unless they set a post-tax price based on what they can sustain with the highest tax rate, which means charging people in lower tax areas a higher price than they would otherwise have charged, just to be able to mention a price at all on TV). Unless the law allowed "between $x and $y depending on your local tax rate" (where X and Y are based on the lowest and highest tax rate for the advertising area).
Most people who want such post tax pricing laws would not be terribly happy with a law that allowed advertising a range like that.