Well first off, have the brakes been serviced properly?
You’ve had the disc’s and pads changed by who?
It looks like you have sticking calipers or slide pins, they definitely do not look like the are working correctly. You should have clean discs on properly working brakes, those look like they’ve either not been used for a while hence the rust or sticking.
BTW rear brakes should last an age, much longer than the fronts, less braking force put on them.
Does it also live near the sea? Salty sea air blowing on them?
The rear discs get less brake effort,
so the pads wipe them less hard,
so they are more susceptible to rust from moisture blowing in.
After a fortnight parked in Gatwick carpark one winter the rear disc rust was bad enough I eventually needed to skim them. The fronts cleaned off on the drive home.
The drivers side looks the worst of the pair. I think the passenger side would clean off with more use. Sometimes the inner facing side of the disc can be worse , check that too.
I had similar with one of my daily drivers discs at the rear, an odd wear pattern, I replaced the set. I think due to little use they started rusting more and more causing pitting, they were 8 years old too with just 32k miles on. Still were very serviceable but as said rust/pitted at the edges.