Well serious question or not - here's a serious and simple answer: Technology evolves over time and that includes what is used as weapons. Why use a weapon that requires you get get within just feet of a dangerous enemy for it to have any affect when you can blast them from a kilometer away with one of these 
Go back to medievel times and I'm sure no-one asked "why are we using swords? why not just beat them to death with sticks"
Go back to medievel times and I'm sure no-one asked "why are we using swords? why not just beat them to death with sticks"