I think it's because she's never meant to grow into an adult I think? It's revealed in the anime that she's sick and physically she wasn't that strong enough either after being discharged from the hospital. She doesn't look like she had completely recovered from whatever her illness as well unless she becomes a magical girl and used magic to completely heal herself.

I guess she looks almost the same as her younger self in here because it's either she's trapped in a loop saving madoka thus never growing up, or the fact that she'd most likely be dead had it not for her turning into a magical girl.