People from Britain are British. People from Spain are Spanish. People from France are French. The reason people from the United States are called Americans is because USA is a dumbass name for a country and calling ourselves anything else would sound really stupid. I can imagine the naming process for the United States being something like this:
Person 1: We're starting a new country and we have to give it a name. What the **** should we call it?
Person 2: Idk. I'm having a brain fart right now.
Person 1: Let's think about it... it's a cluster of states in America...
Person 2: I know! Let's call it the United States of America!
Person 1: OMG! Why didn't I think of that? XD
Petition for people of the United States to be called United States of Americish