Why do everyone thinks that , if you got into YC , your company gonna be a sure bet ?
I don't think many people actually think that. We've said publicly many times that we'd be pleasantly surprised if the success rate was as high as 50%.
(Not just being facetious -- I've heard stories about YCers whose companies failed being hired by other YC companies or otherwise finding good positions as a result of their YC experience, but a few anecdotes is no substitute for data.)