I think that the trouble is that high school teaches people how to follow, so they are conditioned for that. When people get to university-level education and, even more so, complete a masters degree, they have access to all sorts of information which the average person will find very difficult to obtain. The other aspect of this, which I would argue is far more important, is the social connections which are available at the "upper" end of society. It's all an old boy's club and semi-closed culture which most people simply don't have access to.

Leading is a skill just like mining or programming are skills - they can all be learned. Given the recent evidence of life-long brain plasticity, it's reasonable to assert that this is more a problem of nurture than nature.

In short, we train relatively few people to lead, hence the low supply.