Why is it that Garth Brooks's music isn't available to stream via any major music distributor (e.g. Apple Music, Google Play, Amazon Prime Music, Spotify, etc.)?
Garth Brooks has always been critical of the revenue model for artists provided by digital channels and streaming services in particular. Also he disliked the emphasis on songs as the selling unit, as opposed to albums. For these reasons he has always refused to make his music available through these channels.
Apparently wanting to be part of a solution and not just a critical voice, in 2014 he founded his own digital distribution service, GhostTunes, and vowed to give a fairer deal to artists on his platform, as well as allow the artists to sell albums only in their entirety, if they prefer so (although sales per song are also possible if the artists so wish).
You can find Brook's music in GhostTunes, as well as many other artists, both new and well established. It is not a streaming service in the strictest sense, though, as although you can hear the purchased music online (you can also download it if you want), you still have to purchase each song or album individually.
I don't have any info on the overall success of the platform or if indeed artists present there do get a bigger piece of the pie, it would be interesting if anyone on the stack could comment on that.
@Jose David's answer (above) is great and was timely, but here is an update: Garth Brooks has released some of his music (Double Live 25th Anniversary Edition) to stream on Amazon Music.