“The second reason is that leaders are often presented as the finished product.”
I would go so far as to say that in our culture to be a leader, especially a public leader, it is necessary to present oneself as such. To present oneself as being in any way flexible or capable of further growth is political suicide.
Yet, our greatest presidents clearly grew as persons, thinkers, and leaders while they were in office: they grew into the office. Lincoln and Kennedy are particularly excellent examples.
We haven’t had such a president since President Kennedy. I thought both President Carter and, believe it or not, President Trump had that potential, but I was wrong.