It is possibly that knowledge is vast. It is so much that it is very difficult to master the knowledge in a subject or a field.
It is very difficult to become proficient in a subject or topic, it is continuously expanding and increasing in depth (or height) or width (or number of newer specialized areas).
Climbing mount Everest is possible and standardized. The method is physically possible for many persons, if certain precautions are followed and physical fitness is there. Following some persons ahead of us can lead us to the top.
But climbing the mountain of knowledge means learning in depth a subject, it takes years and concentration, mental capability, physical fitness (for mind) and sharpness, memory, and mastery in communication skills. Conceptual and imaginative skills are needed, which are not present in every one.