It’s because Gallatin is a school of individualized study and you usually need to write a compelling essay about how you will use that freedom to structure your course of study (and generally in a way you would not be able to easily do via another school) to get admitted. So applying undecided kind of defeats the object of Gallatin. I have in the past seen people think it’s easier to get in via Gallatin and apply for that reason, but if you’re just doing it as a way to try mirror a CAS or other degree without having to go through those schools’ admissions process it very seldom works.