Guns are handled routinely, and moved into and out of controlled environments, and exposed to dirt, grunge, body oils and salts. They can subsequently dry out and crack very easily, especially if just waxed without lacquer. And once wood dries, it begins to check, or begin showing small surface cracks. Where the wood frequently makes contact with the body it will also haze.
Wood dries out unless treated with a surface finish, like lacquer, to protect against rapid changes in moisture content from moving from a humid environment to a dry one, or vice versa. Drying out and cracking of wood can result from exposure to too much moisture: the wood swells, the fibers take a compression set, and then, when the RH goes down again, the fibers can’t expand evenly, and you get checks in the wood.
Waxing alone with no surface finish does not offer adequate protection. But the wood is still going to pick up and lose moisture seasonally or from being moved into different conditions. even a good, well-maintained finish just slows this down to where it’s less likely to damage the wood.
And, of course, that’s why we put wax or some other heavy coating on the end grain of boards we’re drying — end grain transfers moisture much faster than face grain, so if you slow that down, the wood is less likely to check during the drying process. But once the wood has checked, that’s pretty much forever; you can’t fix it by adding moisture back into the wood.
You can improve the appearance of a rather small piece of wood with checks in it, like a molding plane, by standing the end grain in boiled linseed oil for a few days, so the oil gets soaked up that way. Checks are still there, but this will sometimes expand the wood fibers so that they’re hidden. Lemon oil works well as a treatment because it’s a pretty good solvent for oily finger-and-hand gunk, which is the worst thing for a finish.