Well, the guy has written textbooks on economics, you can take it up with him. :) From the interview:

LEVITT: The very first class you take when you go and enroll in economics — introductory economics — typically on the very first day they put a demand curve on the board. And like every other economics student, when I showed up my freshman year in school and saw that demand curve, I just accepted it. I said, “Well, they put it on the board; it must be real.” And I never questioned anything about demand curves. I always just thought of demand curves as something that exists, like buildings or trees. They have a sort of a physicality to them.

But then when I went to write my own textbook with Austan Goolsbee and Chad Syverson — now this is before Freakonomics, so maybe 13-15 years ago — I actually had to sit down and write about demand curves. And I thought for the first time, “what really is a demand curve?” And I thought to really bring it to life, I think I have to find one, I have to show one to the students.

And I looked around, and I realized that nobody ever had really actually estimated a demand curve. Obviously, we know what they are. We know how to put them on a board, but I literally could not find a good example where we could put it in a box in our textbook to say, “This is what a demand curve really looks like in the real world,” because someone went out and found it.

DUBNER: So you’re saying that the demand curve as it exists in economics and economic literature is kind of a fiction or an invention of economists to explain the rest of the transactions around it?

LEVITT: Exactly. It’s an artificial construct, which turns out to be incredibly valuable for organizing the world and knowing how to analyze problems. And in that sense, who really needs to see one or to find one? But I’m a very tangible person, and I thought, if I could really have an example of a demand curve maybe I could push up the learning of the students.

It took me, I don’t know, 15 years between when I took the beginning economics to when I wrote the textbook to actually think hard enough about demand curves to really understand them. I thought if I could show someone a real demand curve, I might help the students learn about it much faster.

...

LEVITT: Yeah, so we aren’t faking it. I think that’s the wrong way to think about it. What I’d really say is that we completely and totally understand what a demand curve is, but we’ve never seen one. I don’t know if it’s fair to make physics comparisons, but you can imagine something like in the old days when the models had figured out something about protons and electrons, but we hadn’t actually figured out how to literally see an electron. And we knew it had to be there from the theory and then it was just a matter of making the colliders go fast enough or whatever it takes to see an electron, and then it was confirmed that they were there. So, in economics, it’s even easier because it’s not that we have to wonder whether demand curves exist or not, because we know they exist because we define them and they’re there. But I wanted to touch one; I wanted to hold a demand curve, and I had never had a chance to do that until I took Uber and it suddenly occurred to me that here was a chance to hold a demand curve in my own hand.