Well, Wix isn't really file-based (to the user's eye). They're probably just a bunch of entries in a database. It's more that Wix doesn't offer any sort of easy export to a standard format.

Editing in Wix is kinda like Dreamweaver was in the old days, a bunch of WYSIWYG widgets and building blocks you drag around on a virtual canvas.

If they wanted to, they could probably offer a static HTML export of the rendered pages, but as far as I know, they don't.