In another vein, technology could also help us perfectly fit bodies by altering our cells at a molecular level. But if there is no need to move to contribute to the economy, why would anyone do such an expensive thing?
Because it's nice to have the strength when you need it. Also, it protects your body. I developed a bulged disc in my neck from decades of spending too much time at a computer. Muscular balance and variety of movement is critical to maintaining a healthy body. Not to mention benefits like lessening injuries from accidents.
And it is not even expensive to a full type 1 civilization on the Kardashev scale. We just need to hack some genes to not require exercise to develop muscle mass. We already can do it with drugs, and we probably could do it genetically if it wasn't for safety and ethical concerns. As for the cost, I expect a Type 1 civilization to be able to fix genetic diseases as a routine operation, and they could fix muscle atrophy at the same time, if desired.
Muscle mass requires more energy, but by definition, a Type 1 civilization has no shortage of it. And I expect making food out of thin air (like plants do) would be the kind of technology such a civilization would have.
Also because physical attributes are a key component of attractiveness, regardless of their actual utility.
We're not just economic units. We eat, we love, we breathe fresh air, we sleep. Even in a far-flung future, if we are still human, we are still animals.
Because it's nice to have the strength when you need it. Also, it protects your body. I developed a bulged disc in my neck from decades of spending too much time at a computer. Muscular balance and variety of movement is critical to maintaining a healthy body. Not to mention benefits like lessening injuries from accidents.