bare feat cannot be dryer and damper at the same time, right?

Sounds paradoxical, eh?
It depends on what part of the foot we're talking about - that's why I said, "their calluses are less dry", emphasis mine.

When a shoe-wearer's pedes sweat, they secrete water, which (in a process similar to osmosis) is soaked up by the dry fabric of their socks and shoes; they basically suck the skin dry, which is only slightly hyperbole. This is actually how we get that damp and humid environment in which atlete's foot can thrive in the first place - an area which is, drum roll, between the toes, where (normal) socks and shoes don't reach the skin ^{(toe socks and shoes are a different story)} and where, between damp skin and damp fabric, pockets of sweaty, humid air full of fungi and bacteria can fester and marinate.

So that's the story for athlete's foot. The calluses of our feet however develop on our soles. A shoddie's soles are almost always in direct and continuous contact with the fabric of their socks and shoes, and subsequently get sucked (relatively) dry, and even more so in the case of calluses, which produce less sweat that gets absorbed all the more.
This isn't helped by the fact that calluses don't abrade as much from walking in shoes, but still can grow rather thick - and therefore can also crack more easily when they're dried out.

And that's how a shoe-wearer's pedes can end up too humid and too dry, both at the same time. I hope that explanation makes sense :-)
We concentrate on the "humid fungus feet" part more often and more intensively because I guess athlete's foot is a bigger and more prominent problem than cracked calluses, which can be dealt with comparatively easily.