They're required when performing aerobatics, or if you plan to open the door in flight. (e.g. pilots of skydiving planes need to wear one, even though they plan to land with the plane)

But bail-out rigs are much smaller than normal skydiving rigs; the latter would be uncomfortable to wear while operating the plane. A bail-out rig is thinner, shaped more like a seat cushion, and contains only a single parachute, often a non-steerable round (which can reliably open much lower).

Example of typical bail-out rig: https://www.summitparachutesystems.com/pilot-emergency-back-...