Why is the flux density and amplitude different for galaxies than stars?
A galaxy is obviously going to be much more distant than the stars. On the other hand the galaxy contains a lot of stars which will contribute to the overall spectrum.
Whether the galaxy or a star will be brighter will depend roughly on the ratio of $N/d^2$, where $N$ would be the number of stars and $d$ would be the distance to the galaxy.
What your plot has done is empirically remove this factor by linearly scaling the galaxy spectrum so it matches with the stellar spectrum over most of the wavelength band shown.
On the other hand, the light from a galaxy is not only made up from the stars that are within it. There are contributions to the light from (for example) hot gas near any active galactic nucleus and also relatively hot gas in the interstellar medium, possibly associated with bursts of star formation, supernova remnants and nebulae ionised by hot stars. These latter contributors are responsible for the emission lines seen in the galaxy spectrum.