There comes a point in an interview where you're not sure you want the job given what you've seen, and any judgement of the candidate's ability once they've checked out is useless.
You can learn a lot about a person by what questions they chose to ask you, but only if it's not a thing tacked onto the end of a long interview. If they haven't gotten much time to ask you questions, they only have your interview questions to go on. Which you know are not representative of the kind of work you do (but then, why are you asking them?)
So because you only ever talked to them about things that could readily be had off the shelf, and this is the only information they have, they can't know if you're bad at interviewing or some psycho who spends all their time reinventing wheels and then demanding that other people be impressed by your insanity.
The chances of the latter are bigger than you think. So if it's down to you or another opening, they'll take the other opening, and then you've wasted everyone's time.