It's about protecting each other, basic empathy. Doing the bare minimum to and taking miniscule risk to protect the lives of people around you. Sorry that's hard to understand.
It is 'hard to understand' if you get harmed in the process. Hence the jokes about 'doing it for grandma', or 'screwing yourself over with lifelong complications for someone who's already 80.'
"Think about grandma" was the covid version of "somebody think of the children"
Truly, it's the same moralizing appeal-to-emotion but with a different target ("you care about grandma, right? you wouldn't want her to die, right?"). I've always felt it was strange that the argument was (and still is!) largely blindly supported on HN, like haven't we heard this one before?