Exactly this. made even worse on PHEV's and EV's which use regenerative braking as well, the rear discs do virtually nothing. Not surprising to see drums being fitted to the rear of some EV's now.Yours cars are driven too gently, in that condition rear brakes haven't got much to do. Once a week few strong pulls and breaks and it will be
Now this disc looks like the pads is seized in caliper and it need a good clean / lubricate and reassembly...