I think instead of viewing this with the "the customer doesn't understand themselves" lens, you could better view this problem as one of misaligned goals, where

I think instead of viewing this with the "the customer doesn't understand themselves" lens, you could better view this problem as one of misaligned goals, where asking the customer gets you advice how to archive the customer's goals, not the companies'. E.g. in the milkshake examples, customers probably were giving all kinds of advice how to make the milkshake taste better or how to streamline the buying process, etc. And this feedback would certainly improve the milkshake-buying/drinking experience of those who already decided to get themselves a milkshake. However that's not the goal of the company: The company wants to sell more milkshakes, not better ones - it probably couldn't care less what people do with their milkshakes once they bought them. So the companies' goal is probably better served by tracking their customers and finding out how they get to the decision to buy a milkshake in the first place - but that's not really the customer's goal.