Safari (not WebKit specifically; the rest of the browser) relies on OS libraries. Those libraries would have to be reimplemented for other OSes.
I don't mean that that would be an impediment to porting; Apple have done this before—Safari 5 was available for Windows. (And interestingly, vice-versa, IE5 was available for macOS!)
What I mean is that the differences in implementation of these OS libraries would mean that this version of Safari wouldn't be bug-for-bug compatible with macOS Safari; and therefore, testing on this version of Safari wouldn't necessarily get you what you want, if your goal is finding and squashing 100% of the bugs that testing on macOS Safari would allow you to find and squash.
To use a pretty close analogy, it would be like a development workflow for Windows executable that involved running them under Wine in Linux, and never actually under Windows. You've QAed for the API surface, sure; but you haven't actually QAed the software for how Microsoft's own library implementations work (and the bugs that those implementations introduce.)