Some problems are "technically straightforward", where it is "easy" to build but "hard" to find and work with customers. To-do lists are a classic example.

Others are technically challenging. That difficulty is precisely why others don't tackle the problems, but generally customers are looking for a solution and you have a ton of flexibility when working with customers.

To put it in different terms, in some cases "if you build it, they will come" is true. The hard part is determining when it applies.