1) It has nothing to do with the economy and everything to do with whether people love your product and you have product-market fit. Getting to that point takes time and mostly companies run out of money before they get there.
With regard to 2), if that's indeed how VC works, then YC just needs to fund every company. There's no need to do vetting if you are guaranteed to fund the next FB and the earnings will outweigh all operational expenditure on non-FBs.
1) People would definitely love a machine that prints gourmet food for them in their house, but it's really difficult to do that. It's so difficult that the amount of money it would take to make such a machine is pronably greater than all of the money in the world. This is what I mean by "the economy has a cap on the number of new ideas that can be funded"
2) Yes that is how VC works.