Does anyone really think you get infected by briefly passing someone outside?
I don't think many people do think that, no. But I guess if you live in a town or city where multiple people are passing multiple people, the certainty of that is lower.
I think most countries that have made masks mandatory in all public places just did so out of simplicity. I definitely think one of the biggest problem's with the UK's approach has been it's bizarre need to over-complicate everything and constantly change the rules. People get confused and are more likely to bend rules, etc.
In my country, they just made masks mandatory in public at the end of March. And said don't meet up with people you don't live with.
In the UK, it's all "Masks don't work. Okay, they do but only on public transport. Ok and in shops too." as well as the weird support bubbles of 6, unless you live alone then it's 4 plus your age divided by 2, unless it's Christmas then it's 3 more than it was in August if you were outside but 2 less than if you're inside. And the list goes on.
And don't get me started on the slogans! Head, Shoulders, Knees and Toes, Save BHS!