You can define the existence of a market. You can know that there exists some customer with money and a problem you can solve. But that's not the same as being able to say, "Hey, wait a couple of years while I implement this thing. I don't suppose you'd be interested in paying up front?"

Every successful startup has that moment of bravery where they commit their own money and hope that the customer still exists, and is actually willing to cough up money rather than continuing to do whatever they had been doing while they waited. Every failed startup had the same moment, only the customer turned out not to be willing to spend the money.

There are obvious cases where they should have known that no customer existed. But there are also a lot of cases where they simply weren't in the right place at the right time. I think it's a nice myth that the business majors tell themselves that they would always have known beforehand, but it seems more like survivorship bias to me.