All of the greatest leaders that came to the forefront or were going to make a large mark were assassinated, dragged down or did not come at the right time in many generations by the time the 3rd Age came about. There is no one to inspire humanity as a whole. Even the White Tower became bogged down, and that has the largest collection of concentrated knowledge and expertise on the continent, and I think their actions preventing wars is partially responsible for this lack of innovation - out of conflict grows new countries, and the current mission of the Aes Sedai is to maintain the status quo, with the Sisters confusing that for peace.