Thousands of companies build things people want every year. Almost all of them go out of business without any customers. Building something people want is not nearly enough, not by far. You also have to sell it to them, keep them using it, and outcompete other companies in the same space. This takes wildly different talents, and if you don't have those talents, it takes a lot of money.

Saying "just build something people want" is reductionist well past the point of being useless, it's almost actively insulting. Customer acquisition is the death of almost every failed company.