Because no serious app uses either RN or Flutter, and when they do, they basically have two apps in one code base.

Some RN and Flutter developers do a lot of work to mimic the UI on Android and iOS separatedly. They manually recreate.. say.. the new glass effect from iOS, on RN and Flutter. Same happens with Android's design. But it is never the same. Any user (because they use their platform everyday) is an experienced pixel picker and can easily notice that "something is wrong" with the UI in this app.

RN and flutter are great for simple apps that don't need a lot of UI work. They are usually how companies prototype things before they are big enough to move to a native approach, with separate teams, for each OS.

Multiplatform has been a dream for decades now. But it still doesn't deliver. It's similar to how Augmented Reality headsets exist but are really never good enough to catch on.