is it legal to advertise sales prices with sales tax included?

If it is, isn't it funny that you literally never see it?

You'd think in the 21st century where people can shop around on their cellphones, merchants would get smarter somehow.

Online, you often see "free shipping" or "shipping included". Why don't we see these at marketplaces, trade shows, small businesses, ..etc.? Especially when most of these places take cash.

I'm sure people don't like counting change, so what's stopping them from advertising "all prices include taxes, out the door" and keeping all numbers in full dollars? Wouldn't that save both time and money?