Two reasons.

Women want children earlier (because menopause) and are more affected by them (because giving birth) than equally family-minded men. As a result, women are more motivated to prioritise having children/family.

In addition, men derive more advantage from more money/power than women, so they're more motivated to climb the corporate ladder (or take risks and fund companies) than women.

I'm generalising, obviously, so "on average" everywhere.