The issue with the rear calipers is the handbrake mechanism.
You can reseal the main piston fairly easy but removing the handbrake shaft requires a press and various somewhat specific tools.
I've replaced the main seals on rear calipers before, but i've never been totally satisfied with the outcome, whereas a new caliper for £50 just seems much less hassle.
If it was £200 new or £30 for a rebuild kit then sure, i'd be investing the time into doing it myself, but in reality its £20 for the seals, another £20 per caliper for pistons as the old ones are often knackered, then several hours of messing around, vs £50-70quid for a complete new caliper ready to bolt on. Its just not worth the effort.