I've faced this exact situation just recently and it's indeed a bit annoying when you get a persistent interviewer. The universal answer "I'm looking for the next challenge" is so over-used that it's become a non-answer, even in cases like this where it's perfectly appropriate. Let me guess:
- you're not actively job searching
- you are wondering if it might be time to move on whether that's for personal growth, improving your resume or out of financial motives
- you want to keep an eye out for interesting career opportunities
- you don't mind spending some personal time in interviews
It sounds like most of this applies to you and as a result you now find yourself in an interview with a company you may never even have heard of to talk about a position you don't have much info on but simply sounded interesting. And suddenly you get the magic question: why did you apply for this position?
As you'd expect a gut answer like "well I didn't apply" or "it's in line with what I'm currently doing but you'd pay 20% more" is not the way to go here. Instead, you need to acknowledge the reasons why you applied. Here are the core elements you want in your response:
- you're at a point in your career where it makes sense to move on / look for the next [opportunity / project / employer / job]
- the position interested you because of [specific reasons]
- you think you'd do well in this position because of [relevant skills and experience]
- you may have other specific reasons for wanting to work for this company or team: joining an an industry leader would let you work on the cutting edge of your industry, you'd work for/with a former manager who was an amazing colleague or mentor, there's some specific aspect of the work that fascinates you, ...
- [optional for non-profits] you believe in the mission or what the company stands for
What you say exactly will depend on the job, your profile, the interviewer(s) and how the conversation plays out. One example script would be:
Well, while I'm happy with the work I'm currently doing I have been thinking that it might be time to move on. When I was contacted about [position] the job description seemed like an ideal match for my profile but it was the focus on X and Y that made me apply as that's in line with the direction I want to take in my career. My current company does not consider Y a priority but that's something I'd like to explore further and from what you've said your team is looking to deploy Y aggressively throughout your organization, which is something that definitely interests me.