You actually hit on the problem that foldable phones have in common with 3D TVs: both were intended as ways for manufacturers to charge premium prices, instead of being a way to meet a consumer demand.
Yes, they think there is consumer demand, but they are not doing this because there is an unmet consumer demand. They are doing it as a gimmick to charge more. That rarely works in the long run. Unfortunately for them, the consumer segment interested in this is not, for the most part, those willing to pay for a premium phone. I suspect that the market for these phones are mostly people whose target price point is $200-$300, but would be willing to spend an extra $100-$200 for a flip phone. The manufacturers are looking for a price point of $1000 or more.