It's not like the torrid summer season makes people more likely to work, either. (And the range of temperatures at which it's neither too cold nor too hot to work probably varies a lot from person to person.)

This misses the point, since most tropical places don't have terribly hot summers either. The main feature of tropical climate is low variation and year long warmth. For example São Paulo's warmest month is February, with an average high of 82 degrees Fahrenheit. Minneapolis, North America's coldest big city, has a July average high of 83. The interseason spread is about 60 degrees for Minneapolis and 11 degrees for Sao Paulo. So summertime heat in the tropics is not at all comparable.