"then over four person-hours are needed for each person you want to track just to deal with the mistakes from the system"
Which is fine. You've consumed, worst case 200 man hours. The alternative is to search everyone which is an impractically large problem. So you get stabbings at Notting Hill.
At football matches you deploy human spotters, but they are rare, highly trained and able to go to choke points. Human spotters have done much to reduce football violence but aren't a scalable option.
So now consider a Royal Wedding. You've got half a million people lining the route, all within striking range of the couple. Every terrorist, Fathers For Justice campaigner, republican or jilted lover is going to want to have their moment of fame. How do you stop that?