ATC = School Crossing Guard ????
An airport is a big place and an ATCO has several duties; so, may not be watching a particular a/c if/when the crew screws up.
We can be sure that the ATCO on duty dearly wishes s/he spotted the mistaken lineup in time to prevent the accident.
But pilots are big boys and girls who are expected to know their way around and ATCOs are not school crossing guards.