"perished" seals would cause a massive leak, one would notice brake fluid squishing out when pressing the levers...
Over time the rubber can start to swell, causing increase of friction at the pistons, could even snip off rubber fragments which then jam in the clearance... pitting/corrosion/build up on the (exposed) cylinder walls will add up/accelerate this, when then forced into the cylinders/callipers (like when replacing pads) the deposits/debris can also score the cylinder walls...
Long-term operation with worn down pads and/or thin, worn down rotors in bad weathers combined with lack off/infrequent cleaning are contributing factors; not only that the brake cylinder walls remain exposed to the elements, prone to corrosion and collecting deposits, reduced guidance due extended cylinders will also cause increased load/wear on rings and bores/walls...
Generally is age an issue, IMO are about 10 years/100Tkm/60Kmiles a good uptime to plan a general overhaul of the brake system; or when one feels significant increased resistance while pushing the pistons in while replacing the brake pads... normally you just grab the rotor edge before & behind the calliper with your fingers, for pressing the calliper towards it with your thumbs, should retract relatively easy (unless their about to seize or the reservoir is overfilled); any significant force required there is an indication for a rebuild being due...
If this rebuild only requires cleaning and new rings, new pistons too or, worst case: also new callipers, depends on age, condition due contributing factors and how long a potential "issue" had been neglected by the servicing workshop, previous or current owner...
Before pushing the pistons into the callipers they should/must always be cleaned (gentle blow with compressed air) and visually inspected; some even use pipe cleaners to wipe the piston walls clean... pushing dirty, obviously crusted pistons back into a calliper is "deliberate destruction of customer property", at least in my opinion/standards...
As already mentioned is brake-cleaner a no go, it'll seep into the gap, cause the outer ring to swell, yet even make it through to the brake fluid, causing more trouble...