Because she'll always be an understudy, essentially. She can deliver a perfect performance to mirror previous stars, but she won't ever be one herself. There's no distinctly "Watanabe Sarasa's Oscar/Romeo" it will always be a mime of someone else's.
There's an entire arc in Glass Mask about why this kind of behavior is garbage acting for the stage/screen. Also one of the most satisfying.
If you like this series, give the Glass Mask manga a read. It's perhaps the most influential shoujo manga, period.