Women's "market value" peaks at about 20. That's when she brings in the most life, attraction, and -- underscoring all of it -- fertility. Every year after, those degrade a little without exception.
Men's market value peaks much later. Depending on his career trajectory, it can even be a decade or two later, when he has enough momentum and earnings to support her and bring security, future, safety to the equation.
This is simple and universal, and of course it means that it's a very natural tendency for women to be much younger than men in marriageable relationships. Couples can take the alternative route of "we've both young but we'll figure it out" but this requires heavy societal expectations / strong social checks on the wife to stick it out even if his career prospects get off course, living on severe budgets and mere hopes for the future -- and those strong norms are lacking today. Or they can take the other route of "we're both a bit older together when we start" but then the man likely has to sacrifice access to her fertility, which is possibly the greatest sacrifice possible, to support a woman who can't provide a next generation to you. Younger wife / older man is the only equation that's still solid.