The mass school shootings didn’t really take off off until the mid 2000s. So we are really just seeing the generation of constant mass shootings come into adulthood.
When columbine happened it was rare for that type of shooting to happen at a school. The actual rate of school shootings was rapidly dropping by the late 90s too. It is still much lower than it was, only these mass shootings has increased. So, with it being, at the time, a rare event, and shootings in schools on a rapid decline, it just didn’t hit the way it seems to have affected gen z/alpha.