Great man theory describes a real phenomena, it just gets the causality backwards. With lots of people doing lots of things, some are bound to do exceptional things. People who believe they need strong leaders gravitate towards the noteworthy ones, making it more likely that someone who has done exceptional things end up with more power. The more power someone has the more noteworthy acts they are likely to engage in. Then let history forget all but the most noteworthy details, and you have a history defined by great men.