To be fair, this assumption didn't come from nowhere. This was based on existing hunter/gatherer communities. From this interview on it:

"This is the period starting roughly 200,000 years ago when we first emerged as a species that ended about 9,000 years ago when humans started to develop agriculture and settlements. But all over the world, there have been a fair number of people in remote areas who still live this foraging life, and so scholars have looked to them as a sort of window into humanity's past. Anthropologists would go live with them, produce detailed reports, and the sense was that all these accounts pointed to men mainly hunting and women mainly gathering with occasional exceptions."

While this aspect is pure speculation on my part, it also likely fit the culture's perception of women back when the original "men hunt, women gather" hypothesis came out, people tend to want to believe what fits their worldview.