Because the benefit is that we've habituated ourselves to a system where $9.99^plus tax^ is both good advertising, and it means that the vendor passes the tax on to the consumer. As if they can just their up their hands and say "Sorry man, I don't like it either. Here's how much you owe the government." Gas prices all include a tax of 9/10 of 1 cent per gallon for the same reason.
It also likely stems from early on implementation where no one was sure of the vendor actually paid all those taxes after all, so it's a bit of "added transparency" even though it's not really.
Of course, it's 2025, this would be an easy thing to undo, but Americans are creatures of habit as much as anyone else. Try and charge a Boomer $10 even and say tax is included, they will absolutely think you're ripping them off.
That's why I put it in quotes. Its sarcasm to us, but sounds legit to the ignorant consumer.