The more stringent the security measures are, the greater the bottleneck of people waiting to pass through them. The greater the bottleneck the greater the number of humans gathered in a nice contained bunch = potential target.
Take a look at any check in area on a summer bank holiday around the lo-co desks for example, nearly all of whom have large cases with them.
I don’t pretend to know the answer but x-rays, body scanners and other gizmo’s aint it.