Because the vocals are made entirely with an in- app (for free) of the vocoder for Caustic 3!
So if you did not know in the edit window where you can record and edit sound files that is accessible directly from the vocoder you go to the tools section and there is an application on the (just on your Android device not on the iOS version or the PC version) that is in the bottom left corner called Vocality. If you press this you can use any of the installed TTS voices that you have installed on your device, normally at least the Google one is there but there is more that you can buy or pay for. Open it and write the words that you want spoken manipulate the pitch and the speed of the voice and import into vocoder and you can make it sing or say whatever you are too afraid to say out loud yourself...
So again the singing in the song posted is made entirely using Vocality.
Maybe it does not sound so good but it shows you some possibilities and myself never heard it used to sing a whole song before.