Why don't couriers use rear racks?
A big reason to use panniers around town is that it keeps you from getting a sweaty back on a commute. If you're dashing around a city all day couriering you'll get sweaty with our without a backpack, and you're not smartly dressed to start with.

But loading and unloading a rack takes time, and the whole point of bike couriers is to be quick. A few seconds at each end of each job adds up. You've somehow got to get the delivery to and from the bike as well, so loading food into a backpack or parcels into a messenger bag should save time.