They leave because somewhere around the ten-to-fifteen year mark, ambitious women hit a wall no employer can see: the ladder that supplied their direction for over a decade stops supplying the next move forward.
The next promotion used to be the goal. But now it's just... the next promotion. And a woman who can't see her own future can't see it at your company either — so when a recruiter calls with a shiny new role on offer, "somewhere new" starts translating into "somewhere forward."
You won't spot the ‘warning signs’ in her output. You'll spot it in her resignation letter. And by then, replacing her will cost far more than helping her see how she can keep growing inside your organisation.