Think of it this way:

- Hiring is really hard, even tho there are probably plenty of people who want to work at YC

- If you fire people you know are good employees (maybe they were all shit employees?) you're making that harder

- If you're making like 10-20M or more per-employee (seems to be the case for YC) why do you care about having 20 people who you're paying for a bit.