$125k * 400 companies/batch * 2 batches/year = ~$100 million/year deployed just during the batches. On top of that, they often participate in later funding rounds to maintain their 7% stake. Also, it takes a long time for those investments to generate cash. Many of the most valuable YC-funded companies are still private, and even for the public ones I doubt YC unloads its whole stake as soon as it can post-IPO.