At peak times the volume of images that 1 person would have to deal with from 2-3 security lines, would quickly swamp the poor blighter given the task. It's all fine and dandy until they have to inspect one image more closely to come to a decision. Then they would have to stop the other lines they were responsible for checking as well as the line that they had the query on. End result, slower and less efficient screening for the passengers.
Why are we rushing to eradicate jobs that don't need to be? The population of the world isn't getting significantly less, yet some seem to be hell bent on removing jobs, just because they can.