Why aren't there more variations in culture on Earth? First, there is a lot of shared culture across a lot of the Earth, like Abrahamic religions. Second, perhaps the logic of the situation (e.g. men being stronger) lent itself to certain simple gender roles. If so, that isn't why they still exist today in any form. Third, the general logic of memes can create common points across cultures. Fourth, Earth is a small sample size.