Because that was the society’s norms at the time.
Only fringe loonies like Mary Wollstonecraft were recognizably “modern”.
It’s why feminists are now turning on writers like Austen, because they don’t fit their arbitrary 21st Century Postmodernist values and ignorantly don’t even appreciate things like historical context.
That's because she was more Romantic--feelings more important than facts, primitive people are inherently pure, "beauty" is found in the grotesque and exotic--than Christian or Aristotelian, which is where Western Civilization was until about 1800.
Among 1800s female authors, Austen was at the Christian end, Wollstonecraft at the Romantic end, and George Eliot (who was less of a George than Boy George) was somewhere in the middle.