I do not believe that "belief in aliens" almost universally means believing that aliens have visited earth. If you believe people mean this, I think you have a distorted perception because you have only listened to people talk about crazy people. I have asked many of my friends about if they think aliens exist -- some said yes, some said no; the ones who said yes all made reference to drake's equation and similar. I suppose we may live in different social spheres, but still.