Because you will have documentary evidence from the student records of the student having completed those exercises.
Documentary evidence of training isnt evidence of undersatnding and competence,that is the purpose of examinations and tests
A pre-solo written test and a pre-solo navigation test can provide that evidence as well as improving standards.
A pilots flying order book read and and signed by each student can cover other grey areas.
The key is to cover the areas of your backside before the incident because it is too late afterwards