Some cities range across counties. Some cities have taxes that the county itself doesn't, so literally crossing the street into the unincorporated part of town can save you on sales tax. Sometimes sales tax is suspended based on who you are. Sometimes sales tax is suspended on what time of year it is.
Sales tax is complicated. I get why companies would rather outsource that to the register to look up at the end.
It's usually because they don't allow localities to add sales taxes. Here in Japan, sales tax is 10%, everywhere (except for groceries, which is 8%). You don't have to worry about an extra 0.5% from some municipality; it's the same everywhere.
The US gives way too much control to local governments.
Sales tax is complicated. I get why companies would rather outsource that to the register to look up at the end.