Why- the laws of primacy. People feel happier wiith what they are first taught and form a reluctance to change.
Its a pre taxy checklist item for us we dont move until the light is on.
If your taught at a school that has low standards that will be reflected in your approach to flying unless you have a questioning mentality.
Ive asked many instructors why they dont use lights, they dont really know but the usual excuse is that its not nessecarry.