The customer doesn't know what they want
Agile assumes as a first principal that including the customer throughout the development process will align the building team and the customer to come to the same conclusion.
This is almost never actually true. Having built and managed a lot of products I can state fairly confidently that the amount of time the customer has, and clarity about the problem they want solved is orders of magnitude more ambiguous than is required to actually build and deliver something to their satisfaction.
In my opinion successful products, and I mean ones with significant flywheels with customers and viral growth and stickiness are with few exceptions - accidents. It was someone who had an interesting thought and a lot of people for whom the product was good enough for their desire that they often didn't even know they had
You can't write down how to do that, it's like asking a nobel prize winner how they came up with their discovery. It's not repeatable. Some people have better intuitions than others and these are the people who are repeatably successful with products. But it doesn't generalize.
That said, the fundamental problem isn't wrong in theory, it's just naiive in practice.