I’d add one more answer: because it makes for more believable storytelling. I don’t spend my time in real life hanging around with weak women so why would anybody think I want to see them featured in works of fiction?

I’d also say I like seeing strong women for the equal-and-opposite reason I like seeing strong men. When there is a really good male lead (like James Bond or Indiana Jones), the women want to go out with him and the men want to be like him. When there is a really good female lead (like Buffy, or Susan Ivanova (from Babylon 5)), it’s the same – the men want to go out with her and the women want to be like her.