Because to have music you have to get a license from rights holders, and pay for it.
Because to have lyrics you have to get a license from rights holders, and pay for it.
Because to have videos you have to get a license from rights holders, and pay for it.
Because to have your app working on some surfaces (think desktop vs phone vs speakers vs TVs vs cars vs...) you have to get a different license from rights holders, and pay for it.
Because if you want to synchronize or show together some, or all, of the audio/video/lyrics/subtitles/etc. you have to get a different license from rights holders, and pay for it.
Disclaimer: I work at Spotify, and it annoys me to no end that these questions are never directed at the people who actually demand all this money: the rights holders.