They do random spot checks (and do not scan each item in your bag) for the same reason that ticket inspectors do random checks in public transport: there is pro

They do random spot checks (and do not scan each item in your bag) for the same reason that ticket inspectors do random checks in public transport: there is probably some optimum between checking each and every item/person/train (which is very costly) and never check anything at all (at which point a significant number of people might consider payments to be entirely optional).