The successful clients all had someone inhouse they could dedicate full-time to defining a well-structured backlog, grooming stories, balancing feedback from stakeholders, acceptance testing work, communicating with design and development, and generally acting as the unary executive voice of the product.
SomeONE. And the problem is that the role here calls for both executive, rather than committee, function and requires a broad generalist skillset. A good product owner needs to think analytically, write clearly, communicate firmly and negotiate competing needs and priorities. They need to be comfortable discussing technical details, even if they are not themselves technical. They need taste and understanding for user experience, even if they are not themselves a designer. They need delegation skills and a keen editorial instinct.
There are many more people with money to build software than with someone in-house with a matching set of aptitudes and is available to own a project.
Instead we get a group of department heads, with overfull schedules and competing needs. We get people who come to us with ideas for apps they are sure will make them wealthy. We create beautiful software for them, they "launch" in the App Store, and we never hear from them again. We get VPs of Product who spend more time in endless fundraising meetings than talking to their customers and development team.
The real surprise is that the success ratio has been this high.