Having the knowledge available in consciousness and the ability to impart this knowledge to other consciousnesses are two different things.
There are always only a few who can combine both. When they leave, there is often no one left who can match their capabilities. Thus, consciousnesses try to pass on the knowledge who are not able to do so. As a result, their words and deeds are empty.
Since there are many of them, they reach more people with their empty words than the able ones did.
Therefore, the view has become generally entrenched that soul knowledge is just as empty as worldly knowledge.