First of all, foldable phones should fold with the screen on the outside! What's the point of closing it and having to open the phone each time you want to use it. Or even worse, to have extra screens on it so you can see messages or calls when closed??!!! That's just plain stupid. That's why Apple hasn't made a "foldable" iPhone yet.
Apple is about simplicity and simplifying your life, a foldable phone doesn't simplify anything.
My guess is that Apple would make a foldable iPhone only if the screen can be kept on the outside so you can use the phone when it's closed or "minimized" too.