Decision fatigue. Generally you want to reduce sign-up friction as much as possible. The less forms one has to fill-in the better. Asking people to come up with

Decision fatigue. Generally you want to reduce sign-up friction as much as possible. The less forms one has to fill-in the better. Asking people to come up with a question and explaining why they should care is just too much friction. If you have a narrow set of questions the decision-making overhead is much lower.