OK, and what could make the bees more resistant to them? I don't see how they could evolve out of that eating of their body fat thing but I suppose that they could build resistance to the other diseases. Plus, it looks to me like you rarely eliminate the mite completely, just control their population to something that won't kill the hive.
The biggest problem is that varroa vector diseases, particularly viruses. There is never a 100% infestation of mites, literally 1 mite on every bee; the worst infestation I've ever seen was like 25%, so 1/4 of the bees in the colony had mites. Even if 1/4 of the bees are killed by the physical attack of the mites, that colony can still survive, as 3/4 of them still live to fight another day. The problem comes when 1/4 of the colony is infected with a virus or several, and then the bees with mites spread that virus to the rest of the colony. This is how a colony collapses from varroa. We say we hope the bees develop "resistances to mites", but what we really mean is that we hope they develop resistance to the effects of the mites, not the mites themselves, because as you imply, the mites themselves are here to stay.
In that case, shouldn't they build resistance during the treatment process, if they can?
This causes the
mites, not the bees, to develop resistance to the
treatment. Synthetic chemical treatments only take out mites that are susceptible to them, and leave the resistant ones alive, so eventually the whole population in an area is resistant. This has happened with several mite treatments on the market. Some organic treatments, like OAV and formic, don't develop resistance in mites, or at least anywhere near as quickly, since they usually attack the mites more "mechanically", sort of in the same way that the mites attack the bees' fat bodies mechanically.
Edit: I forget to mention something. There is a way to make the bees resistance to the mites themselves, and that is breed bees with hygienic behavior, bees that clean the mites off, actively attack the mites, or pull pupae infested with mites before they have a chance to breed.