It is my impression[0] that a YCombinator startup is a cut above other startups along a variety of criteria (they receive more support in general, they are pre-vetted). It may be perfectly reasonable[1] that forming a startup has lower expected value than a lottery ticket, but conditioned on acceptance to YCombinator, a startup has positive expectation.

[0] For reference I am not currently in SF/Silicon Valley, but my brother is, as are many of my friends.

[1] I present this as a hypothetical, I have not thought about this enough to have an estimation of the value of a `generic' startup.