There are a few possible reasons why 21 Pilots' Kitchen Sink is not currently available on Spotify. It could be that the song has not yet been released to the platform, or that it has been removed for copyright reasons.
It's also possible that 21 Pilots have chosen to make the song unavailable on Spotify, as they have done with some of their other music. The band has spoken out against streaming services in the past, so it's possible they don't want their music to be available on Spotify.
Whatever the reason, fans who want to listen to Kitchen Sink will need to look elsewhere for now. Hopefully the song will eventually be added to Spotify so everyone can enjoy it!