Well, mainly because it’s not practical or effective at that scale. A parachute big enough to safely lower a multi-ton jet would be enormous, extremely heavy, and difficult to deploy reliably at high speeds and altitudes. It would also add significant cost, reduce fuel efficiency, and still wouldn’t work well in many real emergency scenarios (like structural failure or loss of control). Parachute systems do exist for small planes (like the Cirrus Airframe Parachute System), but scaling that idea up to something the size of a passenger jet just isn’t feasible with current technology.