The major reason given is that taxes vary so much in the US by location that it would be onerous for businesses with locations in different areas to print different price tags and advertise prices broadly.
It’s even an issue online because, until you enter your address, the online retailer has no clue what your tax rate will be, and they have to assess tax based on the purchaser’s location. Postal code isn’t always enough, as they can be shared by different cities with different tax rates.
Some areas also vary tax by date (tax free holidays), though I don’t think consumers would care if their total ended up being cheaper than they thought.
A national standard VAT would be the only way businesses might start including tax in price, but there’s no way to do that without a constitutional amendment. States have the power to tax, and they’re not going to stop now even if they receive VAT revenues.
That may be true in many cases, but some people have more sensitive gums, so it depends how skilled the hygienist is. When I last switched dentists, it was the first time I’d never bled during a visit. Prior dentists told me my oral health was good, but my gums were just very sensitive.