Startups are not independent random events because some people are better at making startups than other people are.
I remember reading something that corroborated this [1]. That source claimed that previous founders of successful companies had a much greater chance at additional success, but previous failures didn't increase your chances (ie. you don't, in fact, learn more from failure than success -- at least when it comes to founding failed startups).
Edit adding source: [1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=933932
I have no doubt Y Combinator would still be successful even if their selection method was not very good. (I tend to think about top universities the same way—the quality of Harvard's admissions process is not the core reason they graduate so many successful students.)
But then again, I was just rejected from Harvard, so you might want to take my opinion with a grain of salt.
It would be interesting to know about some companies that don't get to Demo day or that doesn't put out a great presentation
Domain expertise, creativity, intelligence, education, insights from other fields, financial resources, connections to other impactful individuals, personality, etc. Such things significantly affect how events unfold.