[jonp888]

I guess that's part of it, although in this day and age it's trivial to equip each branch with a laser printer and program which adds the local sales tax.

However, there's a simpler reason: for psychological reasons no retailer is going to voluntarily put higher prices on their shelves than they have to, and since unlike most other countries there is no consumer protection law requiring the the actual checkout price including sales tax to be displayed, they don't.