Because they were not ejected from a single point in the Big Bang. This is the classical, and pernicious mistake non-astronomers make in thinking about the Big Bang. As the Big Bang proceeded, small collections of matter began to feel more strongly their own self-gravity, rather than continue expanding with the rest of the universe. This caused the 'flow' of matter to become irregular and the motions very complicated. Overall, the largest systems are still sailing along with the Big Bang, but their constituent galaxies may be moving in complex local orbits of which some intersect causing collisions.
Galaxies do not follow the Big Bang on EVERY scale, only the largest ones.