I was always told that R’ Moshe gave the heter of Chalav Stam being Ok when there was nothing else. Now a lot of people just adopted the ‘minhag’ cause .. why not? If it is OK for R’ Moshe it is Ok for me?!
Besides the fact that CS milk lasts longer sounds like there must be extra stuff in tehre possibly preservatives or other not such good stuff kosher is usually most natural etc…