External conflict on its own rarely stands up as being big enough, emotional enough or important enough. Yes, external factors are important in keeping a story moving, but internal factors are the driving force behind character arc and development, and our pathways to making two-dimensional, imaginary characters human. Real.
When I write strong female characters, I think of it more with the emphasis on strong character, creating heroines and female main characters that are strongly written, fleshed out and three dimensional. The character isn’t strong, necessarily, because of her physical abilities or emotional capacity. She is strong because the characterization is strong and complete.