There are many lessons that cannot be taught.
Many teachings are akin to zen koan.
Attempting to explain the material coherently is not so easy, but a direct physical application proves the point nicely.
If somebody gives you the answer to a koan, do you really understand at all?
Many things in life are only fully understood when you figure them out for yourself.
A student may be given all the necessary training in order to defend themselves yet still falter under pressure.
A student may be offered every piece of a puzzle and never succeed in putting the parts together correctly.
We are all different, and our capacities differ.
The flexibility of the Chinese internal arts allow for this.
They also recognise that some students may just never 'get it'.