Because these springs are made of hardened steel that will rust in no time.
Are you sure they used a spring pin in the mast foot? In my mast’s foot it is a stainless steel pin, not a (blue) steel spring, but it does have a tight fit. So tight it has to be driven in and out by punch and hammer as you described earlier. I stock several boxes of spring pins but I have to store them with a thin film of oil to prevent them from rusting, despite they are kept indoors in closed cardboard boxes that are in a dry and warm cupboard.
Maybe that is the root cause of the problem? Someone at some point in time lost the original stainless steel pin and replaced it with a spring pin? Then, over time this pin started to rust preventing the sheave from running smoothly?