I believe you also suffer from square-cube effects. Take a light airplane with a parachute, and scale up all the linear dimensions by a factor of two. You now have something with four times the drag (drag is proportional to cross section) but eight times the weight (weight is proportional to volume). That means your terminal velocity increases by 40% (proportional to the square root of mass/area). In short, if you take a system designed for a SR-22 and just scale it up for a 737, the jet would descend much faster under the canopy, probably to a degree that it would no longer be safe. The parachute would have to be proportionally much larger still.

And as you say, they wouldn't add much safety because there isn't much more to be had. I can only think of two large jet crashes offhand that would have potentially benefitted (the DC-10 that crashed in Sioux City, and that Japanese 747 where the tail fell off) and that's from a period of decades.

Ideally you'd only need chutes big enough for the cabin and that it would detach and parachute down. Would probably still be prohibitively expensive, but a fun thought exercise.

Then you have the remainder of the plane falling, uncontrolled, and potentially towards populated areas. Without a parachute at least the pilot will (hopefully) be trying to aim for an empty patch, and with a parachute at least the rest of the plane is falling rather slowly.

The pilot can aim for an empty patch, then activate the parachute so that the cabin lands softly and the rest of the plane falls on an empty field. Having the chute system installed doesn't remove any options from the pilot.

First, removing the cabin will cause a large shift in the plane's center of gravity; without adjustment of the flight controls, it is likely to stall, dive, tilt to the side, enter a spin, or some combination of the above, and is unlikely to hit anywhere near where it was aimed.

More importantly, though, the parachute is also meant to provide an escape mechanism for unrecoverable spins and stalls, not just engine failures or fuel exhaustion. In a spin or stall 'aiming for an empty patch' is, essentially, not possible, as the control surfaces of the plane are not usable due to reduced airflow. Additionally, in IMC (Instrument Meterological Conditions), it's not possible to see a safe landing spot. This is an ideal use case for a parachute, but if the plane breaks away as you say, the remainder of the plane will land in an unpredictable location.

There's also the increased complexity involved with ensuring all flight control and avionics connectors between the cabin and body of the plane break away reliably and quickly when the parachute is deployed, and never break away otherwise.

In short, adding a breakaway cabin might make the pilot safer (but might not, given the additional complexity for breakaway avionics and flight controls), but it puts more risk on the public at large when deployed, so it's understandable why such a system was never developed (with the possible notable exception of military ejection seats).

More importantly, the force of the parachute(s) braking the plane would put a lot of stress on the plane structure and it's likely to break apart since it's not designed with sagging or hogging in mind (to borrow from naval architecture where those forces are explicitly modelled due to wave action.)