Why do people say gender is defined by culture?

Where is the culture where men stay home with the babies harvesting food and and sewing clothes while the women hunt and conduct politics and business and war?

Where's the culture where men are valued for beauty, virginity, and youth and women don't peak until their 40's?

Where is this culture where gender isn't based on whether you're male or female? China? Russia? India? Pre-Colombian Mexico? Iraq? Kenya? Germany? No it ain't none of them places.
Why do people say gender is defined by culture?
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