Why don't galaxies for giant "solar systems"?
We don't know your level of education, etc, so don't get too hung-up on technicalities yet. The fact is that galaxies form gravitationally-bound systems and they orbit one another in some pretty complex ways.
More precisely: why don't solar systems against small "galaxies"?
Here is a generalization which ignores such things as captured moons and asteroids, supernovas, planetary collisions, galactic mergers, and density waves for the sake of simplification.