Verifiability is a core policy on Wikipedia, and with time, citing your sources has become more and more important. Wikipedia isn't was it once was in 2001. Articles can't survive on being verified by their own primary sources, for the same reason we don't want Wikipedia to become a dumping ground for advertisers who then cite their own site in an attempt to gain legitimacy. Secondary sources provide a solid ground truth that the subject in question has gained recognition and thus notability. If those secondary sources don't exist, we can't assume notability based on nothing.

Wikipedia isn't Britannica, because by this point it's probably a lot better than Britannica. They were comparable already in 2005,[1] and I have little reason to believe that Wikipedia is doing much worse on that front nowadays, even though they have vastly more content than Britannica.

[1] https://www.cnet.com/tech/tech-industry/study-wikipedia-as-a...