First, compared to a traditional VC, YC is micro-investing in far more startups and doing so faster and more frequently.
Second, YC highly prioritizes founder/team quality over ideas but teams come to YC by pitching an idea. I just read an article yesterday citing evidence that in the last 10-15 years the diversity of startup ideas has declined significantly. This makes sense. Enormous amounts of information about what other very early stage startups are doing as well as which areas are currently being funded is readily available in near real-time. As a serial entrepreneur who did my first startup in the 80s, second in the 90s and third in the early 00s, I can attest we had nothing like this visibility in earlier eras. Ideas which are getting funded and which sound like good ideas in hot areas are obviously going to influence founder idea selection and cause clustering.
Third, YC knows that many startup teams will pivot away from their initial idea once they engage with real customers in real markets. So much so, YC considers it a sign of a smart team working well together. However, like all VCs, at any given time YC has broad sector themes it considers especially ripe for various reasons ranging from new technologies enabling disruption to expected high growth in an emerging sector.
Fourth, is the reason you identified. Having a good enough high-concept idea is necessary but far from sufficient for startup success. In addition to the execution details like go to market that you mentioned there's also timing. Even being 12 months earlier or later can make a difference toward making it. Finally, there's the luck factor. While it's true that quality teams are better able to maximize good luck when it happens and also have a tendency to increase their overall odds too, luck still matters. In my successes we had to get a lot of things right but there were also three or four serendipitous things that made a big difference at important moments. The only way for YC to solve for both the micro-timing and luck factors is betting on multiple similar startups around the same time.