Why aren't galaxies spherical?

All matter in the galaxy has to rotate (not necessarily in the same direction) so that a centrifugal force acts. Without the centrifugal force, all matter contained in the galaxy will collapse into the center of the galaxy due to gravitation. read more

As Rob Jeffries pointed out, there are galaxies that are of spherical and other three-dimensional shapes. There, however, since there is no pre-existing plane of rotation, nothing is causing the matter to collapse into a plane. Therefore, those galaxies retain their three-dimensional shape. read more

I think it is due to the expansion the universe. The image of the known universe shows it as flat, like a pancake or plate. read more