I guess they don't think Flutter is a good experience for iOS apps. When you don't eat your own dogfood, why would you expect others to do so?
I suppose the exception is the web, where React-based rendering and CSS resets have killed off most of the native feel anyway, but Flutter on the web is too slow to actually be usable in my experience. Yes, with a high powered laptop and a JS-optimized browser like Chrome it's acceptable, but that just shouldn't be necessary.
I can see Flutter have its advantages, like on Fuchsia where "native" isn't defined by anything else and on kiosk systems where the application is the only meaningful UI that'll ever be shown, but in apps I'm just generally annoyed by its seeming popularity.