system
Given the following post, respond as if you're commenting on Reddit:
human
Why are US store prices exclusive of tax?

I went to Canada the other day and was impressed (sadly) by the convenience of having in-store prices inclusive of tax. The same thing happened when I was in Europe. Which raises the question: why do US store prices exclude tax? It’s inconvenient for the consumer, who has to know and calculate sales tax when picking up the item/at the register instead of just purchasing an item and paying sticker price with no costly mental calculation.

I am aware that part of this answer may be because other countries have VAT and US taxes also shift from state to state (meaning that a $207 book in NJ may cost $210 in Illinois w tax, for example). At the same time though, it’s literally costless to print tax inclusive price tags, and prices can vary across states.
gpt
It's a combination of the first law of demand (people buy more when prices are lower) and the behavioral fact that people tend to make decisions based on the reported price rather than the full (including tax) price. Companies are able to sell more by not reporting the tax in the price.

As for the legal history of why it isn't included, perhaps someone else can add. I'd guess that companies lobbied against it (from fear of losing sales).