They don't want you to be dishonest. What they want is just to know that there's some interest you have beyond simply a paycheck. If you're there just for pay, you're more likely to not stick around for very long. You may get bored, or you may leave for a slightly larger paycheck, or whatever. Hiring someone is a significant investment on both sides, and they are trying to get some idea of whether you're a good investment as early as possible, before anyone has put too much into the arrangement.

Everyone already knows that the paycheck is the main thing. That's why you're looking for a job at all. The question is "given that, why are you here, specifically". In other words, why do you think that you'll be happy enough here that you'll work effectively and still be here 6 months from now?

My advice is to already have a good answer before you even apply. If you don't, then look more deeply into the company until you do, or realize that perhaps that's really not a place where you'll thrive.

The answer doesn't have to be elaborate or deep. It just has to exist.