Only because we haven’t had 3 years to study. We have lots of data about the impact of the vaccine so far though, and its largely positive.
According to what we have today there is no need to vaccinate healthy young people.
According to what we know, if you have recovered from delta variant you have much stronger and longer-lasting immunity then if you get vaccinated against original Wuhan strain. Yet, EU Covid pass is valid for 12 months post vaccination and only 6 months post recovery.