This is usually because the customization of Android by the phone manufacturer removes the option for equalizer. LG does this on some of their phones, they state that it is necessary to ensure better quality music reproduction.
There are some 3rd party equalizer apps on the Google store that will work with Spotify.
I tried them on my LG G3, but found:
they only have 5 equalizer sliders maximum
little or no effect (mostly worse)
most bring pop-up adverts and other potential spy-ware vulnerabilities