Gradings are based on benchmarks which are necessary for students to aspire to a particular level. They should be within everyone's reach and not just for the elite few, which would, otherwise, discriminate the older person, the less strong and the not so athletic etc. Martial arts is an education for everyone and it is the job of Instructor to ensure students have the best chance possible of achieving their goals. The benchmark should be at a level so everyone tries hard, really hard.
How is the standard set? Instructors have certain expectations but take into account the student’s attitude, ability to learn, physical attributes and class attendance. There are varying degrees of learning so not every technique taught is achievable by everyone, however, the 'try' factor takes the student to another level.
The Black Belt Marathon benchmarks in the past are just a memory. I knew Instructors who sent their students on a 15km run before starting their Black Belt test. They could barely stand on their own two feet let alone do Karate; the Instructor asked me if they should pass. What a silly question I thought.
How hard would it be for the Teacher if a student asks for their black belt? (don't ask the question if you don't like the answer). It's a lot more prestigious to wear a black belt when its given to you because you deserve it rather than ask for it.