There's 12000+ distinct sales tax regimes in the US
https://sovos.com/content-library/sut/state-by-state-guide-t...
Companies serve billions of web pages per second. We can't handle 12,000 tax calculations?
Often not a choice the retailer has. Local laws often require the shelf price be "not tax-inclusive".
Reasons why vary, one common one is that in some instances different customers pay different rates (or none at all).
It's frustrating how much needless friction gets put into the system.
(No snark - serious question, as I'm not from the US, and would love to see the legislation and justification which required that...)
If there's a TV ad for a medium pizza for $10 at a chain they can't possibly know the tax rates for whatever actual store I'm going to go order from.
And the listing on a website won't know until I actually put in my shipping information.