> simply going to the customer and asking them, "Out of these 10 things, if you could have 3 by the end of the month, which would you pick?"

This is only simple for a limited number of businesses. If you're building a product and not selling a service, it's much harder to nail down what is actually important to potential customers. Even talking to existing customers about new features is never a fool-proof way of getting the right prioritization.

Even when you do have a customer you can go and ask, they are rarely a single entity. The person in charge of the buying decisions may think some features are important, but the people who will end up using the product may have other priorities. Even then, prioritization may be hard on their part. If I were building a word editor for you and I came and asked whether word counts are more important than page numbers for you, would you be able to give a confident answer? Would your colleagues give the same answer? How about prioritizing both features over some stability work (e.g. you get both page numbers and word counts, but the product is still a little crashy)?