[sltkr]

To be pedantic, it's still possible for people to modify their behavior based on mistaken beliefs (in this case, that COVID is really dangerous, when it isn't for healthy young people). Though I don't think this explains the actual trend in this case.

[watwut]

Healthy young people still do not want to spread it to grandma or whoever. That is something frequent forgotten by these arguments - not everyone is sociopath and young people sometimes think about other people.