Given that Tierra del Fuego is not the warmest place in the world, why didn't indigenous Fuegians wear clothes?

I don’t know if any historian has provided an answer that you’ll find satisfactory. On this subject, even the best historians seem prone to speculation and flimsy back-formation guessing. Although, maybe that’s a good lesson: not everything in history has a wonderful, detailed, concrete explanation. Sometimes, though very rarely, a somewhat strange, random practice just sticks. A practice starts for whatever odd coincidental or circumstantial thing in history and it stays around simply because there’s no particularly strong reason NOT to do it. You wouldn’t think that to be the case with something so essential to survival though.

In the interests of completeness though, here are a couple of explanations:

As referenced by /u/Lazar_Taxon , Ronald Smith seems to believe that the lack of clothing was a conscious choice to avoid restricting movement as the particular fishing methods used by the Yaghan were similar to those of Polynesian peoples and involved a great degree of agile movement. He explains that this choice stuck around because their stouter, robust bodies meant they experienced little discomfort, likening the situation to that of the Neanderthals.

Dransart, on the other hand, explains the lack of clothing as being an adaptation to their highly aquatic hunting and fishing. Prior to European contact, Yaghan people would quickly dry off, naked by fires between watery excursions. Dransart goes so far as to blame the later spread of illness and fleas on the adoption of European-style clothing. I have no idea if this holds water with current survival advice for getting wet in freezing conditions, but I suppose it doesn’t actually matter. If the Yaghan believed it was beneficial then that’s reason enough.

It’s also important not to exaggerate their nakedness. It’s true that simple pubic coverings was often all they wore, but there’s also evidence for sealskin cloaks, capes of bird skins/feathers, penguin hides and guanaco furs. It’s also important to note that humans often learn to make new tools and innovations by following the guide of nature around them. While Northern Europeans had heavily furred, large mammals to observe and gather fur from, the peoples of Tierra del Fuego had whale and seals as their leading examples of how to survive the freezing, wet conditions. They routinely covered themselves in blubber or oil to stay warm and dry, and used it (whether intentionally or incidentally) to line their cloaks and capes.