Individual stores generally only have to deal with one. Set the prices at the store, and make them tax-inclusive while you're at it. This isn't rocket science.

Companies serve billions of web pages per second. We can't handle 12,000 tax calculations?

If only it were that simple. Some sales taxes are conditional at the point-of-sale. Different customers may pay a different tax rate. This creates a situation where the display price will be incorrect part of the time and may not round to 5c or whatever the legal quantum is.

> make them tax-inclusive while you're at it.

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).