At least in sciences, US universities don't typically even offer a MSc program; people can get MSc by enrolling into PhD, which is typically the only available graduate program, and quitting it after completing the coursework.
MSC’s aren’t offered as much anymore because they had the same problem as PhDs (it could take 4 years to finish one) without the prestige of a PhD at the end. They don’t even guarantee much of a jump start on a PhD. These programs have mostly been replaced by professional non-thesis or thesis-light options, along with “5th year” programs at universities that tack on a few courses and a straightforward thesis to an undergrad program (MIT is famous for these, many universities have copied that).
European universities often have the latter 5th year programs as well where they are considered even more crucial because many of them have only 3 year undergraduates.
Some students leave the school after that point, but it doesn't invalidate their MSc, nor their MSc gets converted to M Phil or whatever. It's not a common thing, but I've seen students drop with MSc mostly because of issues with their supervisors (tenured faculty without much incentive to do research at a good pace).
Edit: I looked it up, and apparently, you're making a blanket statement based on a niche practice in a handful of US universities: https://en.wikipedia.org/wiki/Master_of_Philosophy#United_St...
"Although most American universities do not award the MPhil, a few award it under certain circumstances."
Some of the more academically-focused (private) schools don't have much of an MSc program but they're atypical, and they don't dominate research like they do in other fields.
