This may be because content is moving away from conventional websites and onto platforms.

Reddit, stack overflow, writing sites like wattpad, and content sites like substack, all make content publicly available. There is also a lot of real public content on youtube. These are indexed and show up on a search engine.

But lately, a lot of platforms wall off their content, for privacy (facebook, linkedin), or to allow content creators to monetize it (udemy, patreon, medium). Search engines can often surface content based on e.g. course title, article title, but the in depth content is not accessible so they don't rank.

I think what you are seeing is the end of the open internet, and search results just reflect that.