Because we as hunters are more suseptible to the almighty advertising dollar than hunters before us. Case in point, My father, and my father's father, and my father's father's father, all smoked on the deer stand, but they never had a problem killing deer. They paid attention to their surroundings and most especially the wind. Does that keep me from using scent killer, no... but it brings in the reality check for scentlok commercials.....

Also, the human body cools itself differently than most mammals. The process produces sweat easier and more often, which in turn contains oils and acids that are not typically present in the wild. Add on top of that all the typical odors that our clothing, hair and skinabsorbs from day-to-day life, and you have a potporrii of sorts that the deer know doesn't belong.