Piezo sensors are pressure sensors: they deform slightly and an electric current is generated by the deformation; sound is a pressure wave in a medium; the hollow-body of a guitar is an amplifying device; piezo pickups leverage the hollow-body resonance. At a certain ambient volume level, the guitar becomes very similar to a large unwieldy microphone.
Ambient sounds are always being amplified by the sound box of the hollow-body guitar even without a pickup. Solid body electric guitars do also pick up ambient vibration, but the solid body resists changes in momentum better than the thin wood of a hollow body. A solid body acts as a sink for vibration.
As far as your comment about the distortion method impacting the effect, my expectation is that the pedal "front-loads" the gain a lot more than the computer workflow. Experiment by lowering the pedal gain and/or output and raising the amplifier volume.
The placement of the output source with respect to you is also a potential factor. If you go through the computer to a PA system, and the PA is pointed away from you and 5 feet away, then the feedback result can be quite different than if the output speaker was pointed at you and next to your pedal.
Yelling into the soundhole of an electric acoustic is a well-attested technique by the way.