Why is that? My theory is that the constant hot weather contributes to people not being as ambitious and hard working as people in more northern climes, who must deal with a lot of cold weather. Where I live, I get maybe three months a year of "real" summer weather and during that stretch of time, I just want to sit out in my backyard nursing a cold drink and sitting in my lawn chair. I have no desire to do any work except maybe mow the lawn and throw some meat on the grill. Once September rolls around and there is a chill in the air, I'm more motivated to go back inside and get some work done.
Anyway, that's my theory and I'm sticking to it. Someday, I'm going to retire and will live on some tropical island, with nothing to do but sit on a beach with an umbrella drink in my hand, listening to Jimmy Buffet and maybe some reggae.
Maybe it could be extended to environmental changes which cause alternate periods of plenty and want, which compel the population to plan for the lean periods. Egypt is pretty warm, but it depends on periodic flooding of the Nile for its agriculture to flourish, right? Egypt had to develop a system of agriculture and storage.
There is an old book by Huntington, Climate and Civilization, that argued this point.
There’s also less movement of the sun at the equator. Cold northern climes would jolt the impulse to “be prepared”. Too much cold would discourage the eventual migration—and development of advanced cultures—to the Americas.