I can only really speak to reddit, but I think this applies to all of the user generated content websites. The original premise, that everyone agreed to, was the site provides a space and some tools and users provide content to fill it. As information gets added, it becomes a valuable resource for everyone. Ads and other revenue streams become a necessary evil in all this, but overall directly support the core use case.

Now that content is being packaged into large language models to be either put behind a paywall or packed into other non-freely available services. Since they no longer seem interested in supporting the model we all agreed on, I see no reason to continue adding value and since they provided tools to remove content I may as well use them.