[bdonlan]

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.

[desdiv]

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.

[bdonlan]

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).