perhaps because when we 'knit our brows', --which involves tighten the muscles of our face-- the eyebrows come together like the points of two knitting needles, over the bridge of the nose. --the hair of the brows would look like yarn, too!

the effect would be more noticable on men, (their eyebrows tend to be longer) but it would also be apparent on women who didn't pluck, or otherwise shape their eyebrows.