It's not as simple as adding things up and taking cash. In Greggs staff will probably know the prices for everything but that's impossible for a supermarket, plus there's ever changing offers.
Modern tills don't always open when they crash, staff can't give you change if they can't get into the tills. I remember it happening when I worked for Tesco and we couldn't open the till until IT had remotely fixed the problem.
Loose produce can't be weighed because the scales come hand in hand with the till so they can't give you a price.
As annoying as it is for customers it's worse for staff because they have to feel the wrath of angry customers who can't have their shopping.