为什么不能在fragment中更改文本视图值？

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    TextView textView = view.findViewById(R.id.textView);
    textView.setText("New Text");
}

new Thread(new Runnable() {
    @Override
    public void run() {
        // 执行一些后台操作
        final String newText = "New Text from background thread";

        // 切换到UI线程更新文本视图
        getActivity().runOnUiThread(new Runnable() {
            @Override
            public void run() {
                TextView textView = getView().findViewById(R.id.textView);
                textView.setText(newText);
            }
        });
    }
}).start();

public class MyViewModel extends ViewModel {
    private MutableLiveData<String> textLiveData = new MutableLiveData<>();

    public LiveData<String> getText() {
        return textLiveData;
    }

    public void setText(String text) {
        textLiveData.setValue(text);
    }
}

// 在Fragment中
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);

    TextView textView = view.findViewById(R.id.textView);
    viewModel.getText().observe(getViewLifecycleOwner(), new Observer<String>() {
        @Override
        public void onChanged(String newText) {
            textView.setText(newText);
        }
    });

    // 更新文本
    viewModel.setText("New Text");
}