Nexus and Steam and U.S.-based sites have § 230 protections that prevent liability unless/until they receive a takedown notice or are acting recklessly.

So they basically allow user-generated content until someone issues a DMCA notice. If they do not take down the content at that point, they can be sued. But at any point Disney or any other rightsholder can demand content to be taken down. If there is a fair use argument, the person posting it can respond to the service with a counternotice and demand it be re-enabled, after which the company has a choice to sue them directly.

Based on that, most user-generated spaces do not police content unless they are asked, since they have liability protection and can respond case-by-case to complaints. Also, they may be held to a higher standard if they conduct active enforcement. Sites like YouTube that have very mature fingerprinting and enforcement do so because it helps them have relationships with businesses, avoid large litigations, and sell ads.