The standard use scenario of BottomNavigationView is based on switching between fragments (representing tabs) within the same activity. Since lots of existing apps relate their screens to activities rather than fragments, introducing a bottom navigation bar would mean huge and painstaking rewrites to go from activity-based to fragment-based granulation. In the end, the upgrade may not be worth the effort.
But can BottomNavigationView work with activities rather then fragments? The answer is yes.
This is an example of how to use Android's new Bottom Navigation View component to switch between separate activities.