Sorry, I misspoke (and updated my original question accordingly). I meant for the synths on the Play+. Since the audio is generated on the device itself, why can’t we pan synths?
Ooops nevermind, after some searching, it appears that panning is a setting of the synth itself. Not super intuitive, and unfortunately not something that can be set per step. It seems the corresponding feature request was shot down in Allow panning of synths per step - Feature Requests / Archive - Polyend Backstage.