There is a substantive difference between screening pax arriving at Security and those passing through the Customs channel.
This may be illustrated by the concept of "stock and flow".
Pax arriving at Security are "stock" i.e. once they have scanned boarding pass there is a ton of information immediately available regarding travel history, payment type, itinerary, passport issuing country, covert flags etc. Therefore there is really no need for the minimum wage operatives to worry about picking out certain physical types from the crowd - which they will doubtless mishandle anyway - and this should be done remotely with pax causing concern tracked by CCTV to the Security line then intensively screened.
Pax arriving at Customs are "flow". As they walk up, they could literally be anyone and Customs officers are entirely in the dark about them (except where known individuals are targetted on the basis of prior intelligence). The officers therefore generally deploy "copper's nose" i.e. a career's worth of experience in judging people who are acting strangely (walking funny, sweating, tucking in etc.) and pull over people who ping their radar.
They have a bit of a crap job standing there in the baggage hall - rummage teams have much more fun :-)