Because researchers have been looking for those factors for a long time but haven't found anything significant. Yes, if you give people a pull-up test men will do better on average but there's a shortage of strong examples of more complex skills where there's a significant innate difference. This is especially true for professional skills where there are multiple paths to success relying on many different mental abilities, decreasing the odds that any single innate difference could be make or break.

Porque no los dos? It's possible that "traditional" gender roles evolved, but are still reinforced by behavior even when they are outdated.

The economics of our society have changed both in what efforts are expected from whom, and in what options are granted to whom. That changes some of the billion factors weighing on our constructed social order, but there are still those who benefit from retarding the process.