This is my headcanon, whilst we can control a warframe directly, doign it in the chair probably offers much more support. Maybe some whale music to keep us focussed for long missions stuck inside Rhino, or just somewhere to keep you comfortable and not distracted by Ordis singing as he cleans up after your kubrow. However, when we appear, I really think its better as a void projection rather than teleportation. If the latter, then we'd be an infected mess of infested spores long ago (you never used operator on an infested mission? or appeared in the outer-space parts of broken and infested corpus ships?) Operator cannot be physically present, so it must be a projection of some sort.