Regarding Garth Brooks specifically, the lack of availability of his music on streaming services is entirely his own decision. He owns a streaming service called Ghosttunes that exist /solely/ for hosting and streaming his own music and refuses to make his catalog available on anything else.

Edit: actually looks like he sold it to Amazon a few years ago and it was absorbed into Amazon Music where his catalog is now available. Regardless, Garth owns the rights to all of his music except for his 2014 album "Man Against Machine", so the lack of availability is again entirely his own decision.