Lets put it like this. Not everyone who can read and write is an author, play-write, poet or philosopher. Not everybody who can drive a car is capable of driving a Ferrari, F1 car, Rally car or even a lorry. Not everyone who rides a horse can show jump or is capable of being a racing jockey. Not everyone who can cook is a master chef.
It doesn't detract from all these people are capable of, neither can you say they can't do them because they are not the best.
It is ironic that the best players among musicians can end up in an orchestra pit while others who cant even write musical notation end up writing songs and singing and playing them to a willing public.
You can write bass lines on a computer with absolutely no musical knowledge whatsoever and many are the wealthier for it.
Some of those in that orchestra pit can reproduce perfectly what is written for them and I envy them that dedication and skill but many of them are quite simply not creative and could not write a piece to save their lives. They are not artistic. So how do we try to grade every different level of skill.
We can't