The problem with Flutter is, as mentioned, it does all of its own rendering. That may or may not be an issue for apps, but the second you start truing to support the rest of the ecosystem (watch, widgets, tvOS, even visionOS) you reach a point where a lot of that code may need to be reimplemented.

Another issue is that you're behind the curve again, since as soon as Apple updates the OS/interface you have to wait for those changes to come to Flutter.

The biggest problem, however, it that once again iOS is treated as a second-class citizen as Flutter is predominantly an Android/Material-first development environment. Which is not, as someone else commented, what you tend to want on your most profitable platform.

See: https://medium.com/better-programming/why-flutter-isnt-the-next-big-thing-e268488521f4