Isn’t it adequate to just monitor the outcomes as measured by nutrition indicators?

The IPC explicitly draws, but not exclusively, from nutrition indicators. This is critical from both a practical perspective (nutrition data is not always available and needs to be triangulated with other food security data), and a conceptual perspective (it is well accepted that nutrition is a late outcome indicator of food insecurity). While the IPC uses nutrition data, it also draws from indicators that provide triangulation and give earlier indications that crisis is imminent.