why is it such a secret

I'm not sure about when I first heard about "secret menus," those lists of items available from chain restaurants that they sort of don't publicize. I say "sort of" because it's not terribly hard to find articles and websites that reveal these secrets. Try this, for example.

Generally speaking, the secret menu items represent some different way of putting together the ingredients of the regular menu items. For example, you can go to Five Guys and get a hamburger with grilled cheese sandwiches instead of buns. They sell burgers and they sell grilled cheese. Just put 'em together and you've got a new product.

I'm not terribly big on secret menu items, though when I go to In-N-Out Burger I like to get my burger "protein style" (wrapped in a big lettuce leaf instead of a bun). I once tried their burger "animal style" (grilled with mustard, and lots of extra messy stuff put on it), but was only whelmed by it. I vaguely recall that, a few years ago, Smashburger had deep fried pickles on their secret menu. But when I was at a Smashburger a few days ago, that item was clearly displayed. So I don't know if my memory is faulty (which happens at my advanced age of 137), or if the dep fried pickles got promoted. Other than that, I don't think I've gone off-menu, except for the occasional special request.

But I've often wondered what's the point of having these secret menus? I mean, restaurants are in business to sell food. It seems counter-productive to hold back. I suppose I should do some real research. Write to behavioral economists. Maybe Steven Levitt (author of Freakonomics) or Tyler Cowan (An Economist Gets Lunch) have thoughts on the matter. But I'm too lazy for that right now. Instead, I have a few thoughts to run by you, my loyal reader.

• The thrill. People like to be in on a secret. And when they order from the secret menu they get to feel like they're giving the secret password to get into an exclusive club. The mixed feeling of belonging and status may be something that helps keep customers. I know that the first time I ordered from the secret menu at In-N-Out, I felt an adrenaline rush (once it became clear that they actually knew what I meant by "animal style"). Yay! I was in the club!
• Real estate. Restaurants have limited wall space to display their menu. Maybe listing every last variation would take up too much space and harm the aesthetics. I know that I appreciate the simple menu at places like In-N-Out and Five Guys. McDonald's, Wendy's and Burger King have so many items on their menu that I can't really appreciate the look of the menu. That may seem trivial, but all sorts of factors like that play a subconscious role in meal decisions.
• Prevention of overload. This is kind of related to the second item, and I could easily have been described as part of that item. Behavioral economists (or is it behavioral psychologists?) have found that giving people too many choices can inhibit sales. I can't cite chapter and verse, but I recall reading a study of jelly sales. They found that, if there was one kind of jelly, they sold some. If there were two kinds, they sold more. But if there were eight, total sales were lower. The theory was that when people had too many choices, they froze and didn't choose anything. By pulling some choices off the menu, maybe restaurants are preventing overload.

So what is it? Why do restaurants decline to list their products on the menu? If you know, please share.