Other than saying something like "Because that's the way God made us), here is the Wikipedia link of the eyebrow's function. "The main function of the eyebrow is to prevent moisture, mostly salty sweat and rain, from flowing into the eye. The typical curved shape of the eyebrow (with a slant on the side) and the direction in which eyebrow hairs are pointed, make sure that moisture has a tendency to flow sideways around the eyes, along the side of the head and along the nose. The slightly protruding brow ridges of modern humans could also still play a supporting role in this process in this way it helps them make expressions. Together with the brow ridges, the eyebrows also shade the eyes from sunlight. Eyebrows also prevent debris such as dandruff and other small objects from falling into the eyes, as well as providing a more sensitive sense for detecting objects being near the eye, like small insects."
Another reason is from modern evolutionary thinking - they're there because there's no good reason why not. Instead of 'survival of the fittest', modern thinking is rather more 'what can survive will survive'. A mutation that removes the legs isn't likely to survive. The presence of eyebrows doesn't affect survival in any direct way. The lack of eyebrows could be a disadvantage in terms of the above post, and also for cosmetic reasons. These are totally non-survival factors that affect the ability to reproduce by the obtaining or otherwise of a partner in reproduction. THAT tribe, poor benighted beggars, think the acme of good looks include having a flat back to the head, a sloping forehead, and big front teeth. WE, of course, know better, and look for a proper upright forehead with a nice large nose below it, and a chin you couldn't use as a hammer. We, the real people, do agree with them that eyebrows are essential to good looks. (This thinking could well lead to the modern races of humans, and without the conscious thinking, to things like the peacock's tail.) (Source: regular keeping up to date with New Scientist and occasionally Scientific American. Both recommended reading.)
https://en.wikipedia.org/wiki/Eyebrow
