Some questions, such as this one, may have an obvious answer, which the answer-er may decide to write. In addition, some questions may be based on faulty premises. For example, it seems on this SE site, questions are frequently upvoted, so this question might not have much basis. By answering a question, one doesn't necessarily imply that "it shows research effort, is useful or clear".
Obviously there's no halachik reason to obligate upvoting any question or answer, since its not a significant enough action. If the question was actually good, it may be a nice gesture to acknowledge that. However, it doesn't cross into the realm of "hakaras hatov" since normally it's the answer-er who is helping the questioner. One could always write and answer one's own question anyways. (Unless the question has an obvious answer ;)