Their sole purpose is to turn startups into highly profitable businesses.

So in that context "build what people want" makes total sense to me.

They are not offering you half a million dollars so that you can follow your passion but for you to turn that half million into 100M or 1B.

There's already way too much risk in the execution of an idea to also pursue one that ignores the market and doesn't prioritize value.

I also don't think YC is asking you to build something you don't want to build.

Pursuing passions is a good thing and some people can only be motivated to work on stuff they absolutely love.

That doesn't mean both things are necessarily mutually exclusive, you can build what you and other people want, just be aware at some point in time, multiple times, you'll face conflicting situations where you will have to pick one, and your decision could be life changing not only for you but for dozens, hundreds or thousands of people.