I’m not aware of any public university that charges $50K+ for OOS tuition. Or did you mean $50K+ full COA?
Beyond that, I don’t understand the logic. If as you say STEM programs at “major universities” are all the same, why would it make any more sense to pay full tuition at an expensive private school—even a “very good one”? The sensible thing then would be simply to find the cheapest “major university”, public or private.
But I don’t think anyone actually believes the programs are all the same. Yes, the underlying science and math are the same everywhere, but the quality of the faculty, the quality of one’s fellow students, the facilities, the research opportunities, the support services, career services—these all differ from one school to the next, among public universities as well as privates. Sometimes an OOS public is just qualitatively better than the in-state alternative. And some of the best publics are just pretty darned good schools in their own right.
My own daughter applied to and was accepted at Michigan as an OOS full-pay, with my blessing because I know from first-hand experience with both schools that Michigan stands head and shoulders above our in-state flagship along a number of important dimensions. She ended up choosing a small LAC which in some ways was a better fit for her, but I know she would have thrived and gotten an outstanding education at Michigan for slightly less than we ended up paying for her full-pay private LAC.
Simplistic generalizations like “it’s never worth it” just aren’t very helpful.