Mainly because Sarah hasn't wanted to be identified with that single role (good luck with that, darlin'!) Actors want to be considered for many different types of roles. It helps make them available to play more diverse characters, which simply means that they work more. Sarah playing Buffy wasn't anything like Robert Downey jr playing Iron Man. She couldn't command multi-million dollar pay to play post adolescent demon fighters, so she didn't want to be just identified as someone who could only play a post adolescent demon fighter.
Unfortunately, Buffy was Sarah's role of a lifetime. She is cursed to always be Buffy to her fans. No doubt that's a large part of why she decided to devote herself to being a full time mom rather than keep chasing roles she wasn't ever going to be considered for in the first place.