Jumping around a room exhausting yourself is kind of the antithesis to that.

I'm not sure what games you're playing, but most are what would be considered light to moderate exercise (unless you're playing fitness focused games at a high level), hardly something that is going to exhaust yourself. I'll add that many VR games are standing or sitting experiences (or are room scale but require nothing more than walking).

Nevertheless, there are barriers like the weight and heat of the headsets (and the price) so I don't disagree that it's not mainstream.