Why call yourself an atheist? No one calls themselves aSantaClausists.
We don't call ourselves "a-SantaClausists", "a-Unicornists" or "a-fairyists" because the labels aren't important. "Atheist" is an important label.
Consider this:
- U.S. States didn't have laws of the books specifically preventing a-Unicornists from holding office[1]
- Public schools across the country weren't using taxpayer dollars attempting to proselytize the belief in unicorns to students, to specifically steer them away from being a-Unicornists
- Professing a disbelief in unicorns is not grounds for jail sentences in any countries[2]
- Professing a disbelief in unicorns is not grounds for political suicide, or used as an attack[3]
- People who disbelieve in unicorns weren't as distrusted as rapists[4]
For a-theists, on the other hand, do have to deal with the above. Atheists didn't make the label important. Theists did.
Labels are used for important distinctive concepts. While any of the above, among others, are true for atheists, the label will remain important, and therefore, used.