I obviously can't speak for Byte56, but reviewing the question myself, I'd agree with his assessment that it's off-topic and agree with the reason he chose for closing it:
Questions that are about "which tech to use" are outside the scope of the site.
This is clearly a "which tech to use" question; from the text of the question itself:
Is there any kind of alternative that doesn't have you write your own physics engine?
In your comment you're saying: "'does this exist or not' can't be OT" but "does this exist or not" on it's own isn't a valid question either. "Does this exist or not" is answerable by a single word: "yes" or "no", but that answer is of no use to you or to anyone else. You're not asking for a "yes or no" answer, you're asking for more, you're asking for a recommendation of which tech to use as a follow-on from that "yes or no".
The problem with recommendations of which tech to use is that they're too localized (an answer that's relevant to your requirements may be completely irrelevant for everyone else) and they're coloured by personal opinion (many people will just recommend their own personal favourites).
See also the Meta post linked in the close reason for further discussion on this matter.