You're assuming people are following protocol. Humans undergo lapses in judgement multiple times a day and are just as forgetful. Just the other day, I found myself having scratched my face under the mask while at the supermarket, something I only realised after the fact.
Many people don't realise how much contact spread can occur in just half an hour as well, as seen [0]. This was modelling the spread inside of a dining hall of a cruise ship, but you can imagine similar behaviour in other public spaces.