I'm slow to the game here (sorry Patches, killing your skillful move there...)
The thing is that it's not "1, yes 1."
It's the pub full of people that one guy was in, plus the pubs that those people then went to plus the pubs that all of those pubs full of people went to plus...
Add in every other place that those people went to - supermarket, chemist, concert, church - so the lurgy is well spread by the time one case is identified.
And, if he's got it, who else has it also?
I went back to find the time line of our first, long, drastic, traumatic, lock down. It was two weeks of half lockdown, four, yes four, I'll spell it, f-o-u-r weeks of proper lockdown, then two weeks of lesser lockdown, then trail off. So we killed that-that-must-not-be-named by sitting at home watching series for four weeks*. And then essentially went back to normal life while the rest of the world went up in flames.
Why would we be anti-lockdown?
(Edit: * Not really, I changed jobs, built a boat, found out my kid has ADHD by trying to teach him at home, and since getting him sorted, all at govt expense, his school life has turned around, won a values award end of last term at school, I found out I have ADHD (not at govt expense) because my kid has ADHD and when the psychologist said 'If we don't get your kid treatment his life will probably look like this" and I thought "hmmmm" - which has changed my life around.)