Good point. I think the difference is that in a "closed" environment like OSX there's value in having a consistent look and feel (pretty much set by Apple, for the most part). Not that it's a bad thing, but culturally that's just what has happened.

On the web, which is a more "open" environment (ie look and feel not set by a single company), there's perceived value in uniqueness and originality in design.

That's not to say I agree with the sentiment (I don't mind the generic Bootstrap look either) but I think that's the difference.