Because why should it be? People are willing to pay for it.
This is especially also the case, as Google could abuse some sentence in the YouTube API ToS, which prohibits the separation of audio and video, to get rid of third-party YouTube apps which offered this functionality for free before YouTube Red became a thing. If those had been around, they couldn't really have charged money for it.
There are still a few of those apps available on non-Play-Store-stores, as Google would have had to deal with legal entities to get rid of those (and would have probably lost), but most people never use alternative stores, so that's not really a problem for Google.
This is especially also the case, as Google could abuse some sentence in the YouTube API ToS, which prohibits the separation of audio and video, to get rid of third-party YouTube apps which offered this functionality for free before YouTube Red became a thing. If those had been around, they couldn't really have charged money for it.
There are still a few of those apps available on non-Play-Store-stores, as Google would have had to deal with legal entities to get rid of those (and would have probably lost), but most people never use alternative stores, so that's not really a problem for Google.
One of those apps is NewPipe, if you want it for free: https://f-droid.org/repository/browse/?fdid=org.schabi.newpi...