It simply can't use it because it does not use native UIs, but instead mimics them with its own rendering engine. This approach worked to some extent during the flat minimalist era, but now that Apple has added so many new animations and transitions, reproducing them all has become close to impossible.

At best, Flutter can implement some shaders for the glass'y look of the controls, but something as basic as the Liquid Glass tab bar would require a huge effort to replicate it inside Flutter, while in MAUI and RN it's an automatic update.