That's the nature of startups. Most don't succeed. I don't think YC would make a claim that they are some automatic path to success, just that they help good ideas to get there
Although having a 3% (or greater based on what happens in the future) shot at your startup becoming worth $100M has got to be significantly better than the odds of any other startup reaching a $100M valuation.
And at least 80% of the portfolio value comes from 23 companies - about 3% of companies funded.