I would argue that it's partly because there was less to learn during those periods. I would also argue that it's partly because of how they were taught. When Alexander was tutored in philosophy, he researched the great thinkers of his time and before. He didn't have to know Nietzsche or Hobbes to be an expert in philosophy. Also, most great people were born from wealth. Rather than going to school (which, on the whole, didn't really exist during that time period), 'great people' were privately tutored by the geniuses of their time.