What exactly is gross and dirty about them? I have never thought about them that way (but I don't use them because they generally only have overpriced unhealthy crap).
Anecdata, but a vending machine at the local rapid transit station (one of those two-sided hot drinks / soups and snacks combo machines) had its output chute filled with (very visible and smellable, probably human) feces for a week after(!) calling the service number and only then the owner cleaned it. Mhhh. Didn't inspire confidence in their general cleaning routine, for me.
I have no idea how our machines stack up against the ones from Japan or other parts of the world in the vandalism department, though, in practise and outside of city centres. They might face similar problems, I don't know.
Alternatively, if a company operates for decades, they can make a very, very stable product. Only bugfixes. I'd imagine some of these vending machines are manufactured like that.
Which means, a weekly drive by is enough to inspect, refill, drive on. I used to deliver meats to very rural locations, 10 deliveries over a 300+ km circular route from/back to the packing plant. At 80km/hr, the stops are the big slowdown.
Another way to think of it, is that remote often means little traffic, fewer stop signs, and higher speed limits.
I live 8km from a large city. It's often far faster for me to drive 15km to a corner store in a small nearby town, because I can do 120km/hr the whole way there almost, on my rural road, with no stop signs or lights.
Drive to the city, there are 5 stop lights before the first corner store, that's each way, so chances are high to hit a light. And even one light is 2 or 3 minutes delay.
So rural servicing and deliveries can be quite fast and efficient.
I wondered that myself, after seeing some unlucky porter hauling cases of tea up to the vending machines at the top of some mountain temple (maybe Nikko?) on foot.
It was like something out of Death Stranding. It defied physics, looked absurd and would have killed him if he slipped on moss.