What is the difference between using viewBinding and findViewById() when setting up a cast button?

I have a cast button in a fragment that I'm initializing using the Cast SDK's built-in CastButtonFactory.setUpMediaRouteButton function. When I call it like this:

CastButtonFactory.setUpMediaRouteButton(getActivity().getApplicationContext(), viewBinding.contentButtonMediaroute);

it fails somehow and the button isn't clickable. When I switch it to this:

CastButtonFactory.setUpMediaRouteButton(getActivity().getApplicationContext(), getView().findViewById(R.id.content_button_mediaroute));

it works properly. From what I understand, viewBinding is supposed to be the better alternative to findViewById. Why isn't the first call working?