Educating, finding, hiring, training, cultivating and retaining talent is extremely difficult. Companies recruit at colleges, where students have worked hard enough on their credentials to gain admittance and even harder getting good grades and joining clubs and participating in activities. They will identify a few good candidates. They will recruit these candidates, along with every other relevant company. Making offer letters, a few will accept. These will be trained for months, then cultivated for years. Now you've got to get these people to work together in teams, and some of those teams will work out and some will not. Most workers will leave in a few years, some even sooner. Some will grow, advance and go on to become leaders. One will go on to become the company's CEO. The funnel chart on this system is daunting, going from millions of students to one CEO.
On the other hand, Y Combinator will take anyone with a convincing video, the 'right look' (see: Moneyball) and attitude, and gung ho fervor. The program teaches these teammates to work together to build and ship experiments quickly and get real people using them, and then to adapt to new data to conduct new experiments. Upon graduation, the teams get more money to continue conducting experiments as they seek their market. Most are acquired in 'acquihires.' Buying a working team bypasses the lengthy process outlined above. Huge value to the acquirer, and contracts/incentives tie the team up for a few years.
Bottom line: these acquihires are profitable for the acquiring companies, the founders, and Y Combinator. Making the acquihire machine turn does not require an idea, a cohesive team, or much of anything except the ability to successfully complete the training Y Combinator provides. Some companies go on to find a market, the rest get the degree, a bonus and a job.
YC training is boot camp. It is the new Harvard, formed to match post-web realities.
Or something.
* Not asserting Y Combinator discriminates, but any time 3 people in a room make a decision without a lot of hard data on the candidates' performance at that task... bias must enter. This is one checkpoint in a new system.