The reason usually mentioned is that the labels are produced centrally or some such. Though "They know the price to charge at the till’ might be slightly off when the tax is calculated on the transaction as a whole rather than on a per-item basis (i.e. rounding shenanigans). That seems like a totally solvable problem to me, though.

I took my wife to meet my parents and had to remind her when we went shopping that we had to add tax to everything (and tip in bars/restaurants/etc.) Some things looked cheaper than in Japan until tax (especially at that time when the exchange rate was awful).