tbh, I think people assume that behaviours cluster. Also, it's about "do I trust you to be safe" rather than any actual logic about vacc effectiveness.
So if I'm fervently pro-covid-vacc, and if you are vacc'd, the logic goes, you are probably my type of person who wears a mask, avoids big gatherings, avoids anyone who might have covid. I assume all of that goes together. I like you so I trust you won't bring me covid.
But if you aren't jabbed, then I assume you never mask (wtf is it with people thinking masks make such a difference), go to wild raves, and make no effort at all. You obviously don't care and this means you are dangerous.
In this logic system, a parent nurse of 5 children daily attending school with own social lives, only some of the kids vaccinated, who worked with potential covid patients daily but followed guidance about PPE & obviously had jab = "safe" even though they are constantly exposed and highly socially connected.
Joe Biden would be safe by this logic (highly socially connected & exposed to people, including big groups, but is vax'd).
In meantime, random stranger who barely gets out, orders shopping online, has no friends and is unjabbed = "highly unsafe." This person is twice as dangerous if they espouse any vaccine hesitancy publicly. 5x as dangerous if they are publicly anti-vacc. "I don't like what I know about you so you must be dodgy."
My elderly parents are very covid fearful & I am amused at their activity & socialising choices. It's all based on who they trust.