This is a fairly common sentiment, and therefore one that merits a brief reply – brief because the answer is really very simple. No one argues against the Easter Bunny because otherwise educated and intelligent adults don’t believe in the Easter Bunny (much less base their social and political views on what the Easter Bunny demands of them).
At any rate, atheists are arguing about something they believe exists, namely, belief in God. And plenty of skeptics – both atheist and not – do argue against other beliefs which they regard as erroneous, including that in ghosts, ESP, flying saucers, mermaids, and so on. Ken Ham may be surprised to learn that there are even books, websites and magazines devoted to such topics!