The answer is rather simple. By the time that Eskimos (and even ancestral Siberian populations) arrived in the far North, humans were relying on clothing rather than on body hair for insulation. No one would have frozen to death because their body hair was thinner than others, so there was no selection on that basis. For tens of thousands of years humans have been adapting to the environment through cultural changes, not via selection. (This is not to say that human populations don’t show any adaptations to different environments, just that these are less prevalent than cultural adaptations.)
Eskimos, like most other populations of Asian origin, have relatively little facial hair. But whatever benefit a thick beard might provide to males for facial insulation might be offset by the nuisance of moisture freezing on the hair around the mouth.
An old friend of mine - Korean War veteran, that old - told me about an exercise he took part in, an army expedition across northern Canada before he went to Korea. They wore sealskin boots and coats like the Inuit; he said the boots were so hot and waterproof that the callouses peeled of his feet even walking around in serious sub-zero weather. So I’m going to go with one of the original answers - the advantage of body hair was not sufficient to result in it becoming a positive survival trait, since they could make more than adequate clothing.
Remember, evolution works on differential reproduction - those with better traits are more likely to survive long enough to breed and raise offspring. Obviously a incrementally greater amount of body hair was not a significant advantage vs. very adequate clothing in the short time (what, 10,000 years or so?) that the Inuit have been in an arctic environment…
Consider this versus beards - humans moved into Europe about 50,000 years ago. Perhaps in those ice-age days clothing was less adequate and the risk of men hunting getting frostbite and infection of their chin was greater than Inuit who wear much more complete hoods and face covering.
The Eskimos (which include several groups in addition to the Inuit) seem to have split from the Aleuts only about 4,000 years ago, but their ancestors were in Siberia long before that. However, northeast Asians, including those from very cold areas, tend to be relatively hairless (except for the Ainu).
However, groups that seem to have been in tropical areas for even longer like Papuans are just as heavily bearded as Europeans.
If selected for at all, I think differences in human facial hair and head hair are much more likely to be due to sexual selection than environmental adaptation.
I believe that one anthropologist said that the reason humans have long head hair is to have something to put ribbons in. And I don’t think that is entirely false.
Look, we know that Inuit and other far-north-dwelling peoples do have multiple adaptations to their environment: altered average limb proportions, nasal bone and sinus changes, alterations of fat metabolism enzymes, different distributions of brown fat, different basal metabolic rates, etc.
So anyone asking ‘hows come they don’t got fur’ is ignoring all of the genetic differences that they do have in favor of one particular one that they don’t. It also presumes that massive amounts of body hair would lend a survival advantage and would not in fact be neutral or even detrimental.
Ultimately, the answer is ‘because a trait for hypertrichosis never arose that provided a reproductive advantage’. This captures both the fact that hypertrichosis needed to be present or arise at random in the population in order to be acted on by drift or natural selection (which is not guaranteed) and the fact that such a trait would have to provide a reproductive advantage (not a certainty) and/or go to fixation in the population by pure chance (also not a certainty).
Humans with congenital hypertrichosis often display abnormalities of the gums and teeth. Increased body hair may lead to increased parasite load. Increases in body hair caused by sex hormone alterations may be associated with lowered fertility. Maybe thick fur means that you spend time grooming that would be better spent on finding food. It’s tricky to make a big change in hair production that doesn’t have knock-on effects on other systems. If it did arise in the population, there is good reason to think that increased body hair could be associated with some other effect that would decrease fitness. That’s not to mention the added layers of sexual selection, cultural practices like shaving or wearing clothing, and other factors.
Evolution does what works, not what is best. Our windpipe and our esophagus cross, that’s a major design flaw, but it doesn’t keep us from reproducing, which is the goal of evolution.
Perhaps better to say that evolution moves very incrementally toward what is best. Therefore, steps that would have to be very binary as opposed to marginally different, like switching whether windpipe or esophagus was in front from top to bottom, are very difficult for evolution to accomplish. IIRC there are other tube structures in the body that travel ridiculous differences to get past one another because that can happen incrementally whereas switching which one is on which side can only happen in a single step.
By the way, given that northern indigenous people have to be clothed, a better test of the fate of body hair would be whether hair is helpful inside of clothing. Opining from my own experience of having a lot of body hair, I suspect it is unhelpful generally.
It would be a problem with respect to body lice and other external parasites, which would have a better place to hold on/hide out. This would especially be a problem where bathing would be impossible for much of the year.
