The answer is that it’s more practical than any alternative.
Copyright holders can’t sue Reddit for selling access to copyrighted content (before Reddit receives a copyright claim) because there is no way Reddit could reasonably distinguish between original and copyrighted content. Reddit users violate copyright law and the ToS in submitting copyrighted content, and Reddit is only required to take action as they are made aware of the content’s copyright status.
It would be trivially easy to to circumvent Reddit’s ToS otherwise: I could create some original content, sell my copyright to a friend for $1, and immediately put Reddit in violation of copyright law by submitting the content to Reddit. My friend could go after Reddit, and Reddit could go after me, but my friend would likely get more out of Reddit than Reddit could successfully get out of me.
It’s the same reason publishers can’t sue Cloudflare for hosting a piracy website unless they refuse to take it down, nor can they sue Facebook for ad revenue earned from banners placed next to a copy+paste of a New York Times article. The content providers do not knowingly/intentionally violate copyright law, and they make reasonable attempts to prevent/rectify it. Without such limitations on legal standing, the internet becomes a way bigger mess than it already is.