There is no way to verify content that is hosted on reddit's CDN, anyone could make any claim about OC at any time. The only way IDs happen is in comments, and that comes long after the damage is already done. Reddit used to allow external links, but now only a few subs do it (None of the default subs allow external links) because it's common practice on the site. Regardless of linking to the OC, it's still hijacking views from the original creators and undermining content creators. Reddit does not pay original content creators for their views... If Reddit had to pay OC creators for views fairly, they'd likely not be as popular nor profitable as they are now.

I think because youtube can earn creators significant money, it becomes the a primary source of spam on reddit and profitable youtubers can flood everything out with voting rings.

And that is the stuff behind subs blocking youtube and the strict self-promotion rules.