Because bundling APIs in the SDK has traditionally led to
more fragmentation, not less. Hence the androidx libraries and now Compose, which creates an API surface that mostly works without having to think about OS versions.
I talk with iOS devs often, and most of them do not use SwiftUI because the feature disparity is massive across iOS releases. Bundling features into the Android SDK would be about 1000 times worse because Android OEMs don't update their devices like Apple does.