I don't mean to be call(o)us, but I don't have any!
Way back in high school I can remember a bandmate proudly introducing me to an upright bass player who could hold a cigarette with the filter-end against his thumb while the lit end seared into the rawhide-textured skin of his right hand index finger. Apparently he felt no pain, and somehow I was supposed to be impressed by this G.Gordon Liddy-like act, but no...Uh-uh…Feh!
Sitting in a tree during a lightning storm? That's impressive, but I digress……
Despite a personal preference for fairly high action & heavy-ish string sets (both acoustic and electric), in all the years I’ve been playing I can’t recall having much callus build up on my fingertips, except for a small spot on my left hand pinky. Perhaps it’s because the plain strings don’t have windings to ’sand away’ the sloughing skin cells? It’s a mystery.
I took up guitar in the early 1960’s when string sets were starting to be marketed in different gauges, but there was a time when most strings were fairly heavy gauge "Black Diamond" type steel cables. Couple that with a little too much neck relief, non-adjustable truss rods, a scarcity of knowledgable repair people, no internet, and the Manly Mantra: "Just suck it up and press harder until you get some calluses!”, and you end up with several generations of players believing that’s just the way it is.
Somehow I’ve never had this issue, and simple observation says it’s because my guitars almost always have large fret wire: Dunlop 6100 to be exact; Sometimes stainless steel, but always 6100 no matter the alloy. So how does that relate to still-malleable, though not exactly baby-ass-soft fingertips?
Easy: Less contact with the fingerboard. I only have to press the string far enough to contact the fret crown. I can remember my good buddy Michael Savitt bringing a 1956 Gibson Les Paul Custom 'Fretless Wonder' over to the house. It absolutely lived up to its name, because apart from 22 silvery lines I was left wondering where the frets were! Unplayable? You bet, but boy are they worth some heavy coin nowadays, especially with the staple magnet neck pickup.
Just don't re-fret it and ruin its originality. God forbid it should actually be playable.
It stands to reason that if you’re pressing the string against the fret at the same time that the fingertip is being pressed flat & stretched against the fingerboard, you’re creating more friction. Couple that with sliding on the strings, sweat & dirt buildup, etc, and it adds up to your fingers needing to build a protective layer.
I don’t have any hard proof as to why most manufacturers use (what I consider) smallish fret wire, but the guess is that it’s their way to counteract intonation issues caused by the "death grip” that so many players develop while learning guitar. Too much pressure on the strings will cause any fretted note to go sharp, and the higher the fret, the sharper the possibility, so to speak, and players won’t buy something they think can’t be played in tune.
Back in the 1990’s, while doing retail, I lost count of how many people would return a guitar because “It won’t play in tune”. Routinely I would tune it, play a little to clearly demonstrate that it played in tune, and ask “How’s that sound?”. “Well it sounds fine when you play it, but I’m returning it because I can’t play it in tune, okay?"
There’s just no way it could ever possibly be operator error, right?