Actually, simple. You see, the Halo games and novels only cover a very tiny part of a galaxy. As we Halo fans know, the UNSC has Earth as its Homeworld, and all of its other planets are very close by. Now, if we assume the Covenant also make their homes nearby, it could rationally be said that the part of the galaxy occupied by the two races could be quite small. Now, if this part was located in the unknown regions of the Star Wars galaxy, it could be said that the species if human in the star wars galaxy was actually a group transported off of earth by the Forerunners before activating the Halo rings to try to keep then alive, while giving them superior technology. It could explain how humans were so widespread through the Star Wars galaxy, and why earth was nowhere to be seen...Thereby having the two halo races right under the noses of the empire and rebellion all along.