I've been thinking about this quite heavily, but if we speculate that man was created by Lorkhan, this explains a lot of things. If this is true, then men are the only ones capable of becoming Shezarrines and it may be possible to create artificial Shezarrines, as each man would have a miniscule part of Lorkhan's soul. If one were to gather enough of the souls and combine it with their own, they may become Shezarrine. Perhaps this is why black soul gems can only contain the souls of men and mer, as both of these have some divine aspect to their soul.