The bigger the vehicle the more altitude that is needed to deploy chutes. The bigger the vehicle the proportionately heavier the chutes need to be because they need to spread out more and they need to be stronger.

Furthermore, most commercial aviation disasters occur at low altitude and the ones that happen at high altitude tend to be too catastrophic for chutes to be of value. (If something bad happens at high altitude that doesn't destroy the plane it tends to make it back.) Off the top of my head I can only think of three crashes where they could have used chutes--two where the hydraulics were damaged by high energy events (and would the chutes have worked properly??) and one fuel failure due to hijacking.

I’d imagine it might be technically possible but not practical. The weight required for ballistic parachute systems is substantial. On small aircraft (such as Cirrus and Icon) it reduces the useful load, and thus range/payload, substantially. Range and payload are very important aspects of commercial passenger aircraft.

Further, commercial passenger aircraft are already very safe due to system redundancy not practical on smaller aircraft. Would a ballistic parachute system help with many accidents in this category? I would be willing to bet not. Ballistic parachute systems are not a magic bullet - they require substantial altitude/time to deploy (as much as 900 feet in a spin, for example). Many substantial aviation accidents happen during takeoff and landing below or near these altitudes.

Would you pay a multiple of your current airfare for an extremely small (practical) reduction in travel risk?

Short answer, no. Mostly due to weight, speed, flight altitudes and different certification criteria.

I don't see why not but passenger aircraft are incredibly safe and the pilots are usually very experienced with at least in co-pilot. Even more so if you exclude war and runway related accidents which wouldn't be helped by a parachute.