It takes time to develop good judgement. Many ideas that seem initially appealing turn out to be tarpits that ensnare founder after founder. And some ideas that seem crazy at first are actually good. And many ideas are bad initially, but "ripen" as technology changes.
For example, WebVan was a huge failure 12 years ago. But today Instacart is providing a very similar service, but in today's context they can do it without burning tons of capital.
Part of the benefit of YC is that they see so many startups that they can develop keen intuition into which ideas are most likely to work. Not perfect intuition. But definitely better than the average first-time founder.