I dont understand where your view comes from. I've never seen a gym with a cold plunge. I cold plunge in natural bodies of water or my back yard, and I do it whenever I want - intentionally separated from my strength training.
It's rarer, but it's the same type of amenity for a gym that has a sauna. Not all gyms have saunas either, but a certain type of gym does.
Frankly, I don't understand your view either. The vast majority of folks do not have access to a natural body of water that is cold enough, especially not year round. And having a plunge tank available at your home is a considerable expense, either in recurring ice cost or in the type of machine that can actively cool water.
The lowest barrier to entry is having access to it at an external facility. I lived in an apartment where that had a cold plunge next to the regular pool (which was also next to the apartment gym), for instance.
