Single Stage to Orbit (or any type of reusable launch system) invariably adds complexity and weight, which in turn eats up any gains by being able to reuse stag

Single Stage to Orbit (or any type of reusable launch system) invariably adds complexity and weight, which in turn eats up any gains by being able to reuse stages. It just isn't worth it to add complexity unless you go whole hog and make the entire thing SSO.