No, it is not possible to change the policy to force the volunteers to create Answers instead of Comments. The volunteers would react by saying "Get stuffed! I am going to go elsewhere!"
The volunteers contribute the way that they feel best, when they have the time and inclination. If they happen to feel most comfortable adding a suggestion in a comment rather than writing up a complete wrap-up solution, then we thank them for what they did take the time to contribute, rather than nagging them for not taking even more time.
It is common that the person asking the question does not explain the situation well, and that even if they take the time to say that the situation is resolved that they do not take the time to explain what they were really trying to do and what actually works for them.
It is very common that people ask for assistance with homework or a project, and that the best thing that the volunteers can do is to guide the person to a solution without ever posting a complete solution. Not posting a complete answer is often deliberate in such cases.
Sometimes the volunteers just can't be bothered providing more than a brief outline of steps that can be taken towards a solution. I know that is the case for me; sometimes I can only muster enough interest to write part of a sentence. If the policy were that I had to write up a complete solution then in such cases I would just post nothing.
We get quite a few cases where the person just copies an assignment, or just says that something needs to be done, without asking a question. No-one owes such people a complete write-up solution.