Why are we frying in the oil, but not in any other liquid? (besides the temperature)
We do cook over 100C in a pressure cooker, but not enough to brown food, as this would take very high pressures. It probably wouldn't be safe or economical at home, and doesn't have enough of an advantage over ovens (which may use steam) for it to be worth doing industrially. 

Solubility is a big factor, for example salt dissolves in water but not oil, and capsaicin (chilli heat) dissolves in oil but not water. Hydration is important to the texture of many foods (starch, for example). Cooking in oil reduces hydration, while cooking in water increases it, so I doubt you could make chips (fries) in water even at 200C. New techniques would need to be developed unless this extreme pressure cooking was just a way of speeding up existing methods.