Because screening leads to interventions when it is the data used for deciding whether to do an intervention. If a screening procedure leads only to interventi

Because screening leads to interventions when it is the data used for deciding whether to do an intervention. If a screening procedure leads only to interventions that would be made in time anyway, it doesn't provide a benefit. If it leads to extra interventions and no drop in mortality, it may be a net negative. E.g irradiate enough people to find more cancers and not improve outcomes, and some of your screening may lead to cancers.