Interviewers ask these questions to ensure the candidate they choose will remain in the position and not jump to a different one that would suit their experience, skills, and abilities better.
They are looking for candidates who are genuinely interested in the role (as opposed to those just looking for employment and will move on if they find one more aligned with their experience).
It would help if you showed in your response that while you have higher qualifications than the position requires, you are still interested in the role.
As an example, you can refute the premise and note that you think you have the specific qualifications the job requires. Or, you can describe other motivations for taking on this role, like an improved work-life balance or the opportunity to mentor others.