Staring down at a zipper, it makes little sense that this object's two sets of teeth would line the primary means of egress for one's penis during everyday bathroom use.
The original American zipper brand was Talon, for crying out loud.
Are there no alternatives? Of course there are. Zippers were not even in common usage until the 1920s, we find in Robert Friedel's study, "Zipper: An Exploration in Novelty." In 1937, a zipper company memo held, "Retailers were made to worry that they could be held legally liable if a man injured himself with the newfangled machine on his trouser fly." Nowadays, button-fly pants abound, selling alongside their more dangerous brethren. Also, velcro exists. We don't need zippers.
Perhaps zippered pants remain in circulation because harm to one's genitals only exists in jokes or urban legend. As University of Utah folklorist Jan Brunvand would have it, "[F]olkloric zipper stories, especially stories involving troublesome zipper flies on men's trousers, became part of the cultural history of the product."
Brunvand continues, "The possibility of a man zipping part of himself into a pants zipper fly must occur to many men." But really, who would believe that this happens?