Why would artists not put their music up on one of the most well-known platforms that allow people to significantly support them by purchasing a copy of their music.

I legitimately don't get it.

I don't necessarily believe that Spotify is necessarily worse as a way to make money from their music (I think ad-supported and subscription "bundling" services such as Youtube and Spotify probably result in more money going to artists than all the options for purchasing artists' music piecemeal, like Bandcamp), but artists should definitely make their music available somewhere for fans to purchase regardless

Substitute Bandcamp with Google/Apple music or whatever, the point remains, one can use Spotube and choose to support artists buy paying for their music.

I don't think most people are actually doing this though..