Presumably you don't schedule an interview every single time a recruiter contacts you, so there the question is more of "what made you say yes to interview with us?"

From my experience that answer also means you won't be going forwards so it's just a waste of everyones time. I guess you're supposed to feign immense interest in this cold-call interview.

Yeah that just drives me nuts. If you're recruiting, you're trying to sell me on the position.

Imagine if we did this for other things.

"Why do you want to sell me this apple?"

"Be... cause... I grow apples? To sell? And you said you want one, and are holding money?"

Hilarious. There's a car salesman joke in there.