“Why is every country song about horses, broken relationships, or substance abuse?”
I have never really heard a country song about horses, and I don’t think you have either. Actually, never about substance abuse. Yes, broken relationships, and that crosses all genres.
This country song is about California and concepts asssociated with it.
A simple search online shows scores of country horse tunes. Even if the list is off a bit, the point remains. It’s like the physics club in Breakfast Club: Well, in country songs involving horses we-we talk about horses, properties of horses. https://www.wideopencountry.com/songs-about-horses/
Substance abuse is all over this genre: https://www.ranker.com/list/country-songs-about-drugs/ranker-music, https://www.ranker.com/list/best-country-drinking-songs/ranker-music.
And yes, the singer in California is breaking up with a municipality.
Now, I get the love some have for this genre. Different strokes for different folks. But there’s no denying the core of country.