Nashville is and was indeed a magnet for all sorts of singers wanting to make it big. But the Nashville record companies run a tight ship and have for a long time. It’s not unlike pop producers, they use talented artists but pump out music with the same proven formula over and over. When you sign up they want to you record with their musicians and they pick the songs. Like Jamey Johnson said in “Between Jennings and Jones:”
Also for record companies, I would imagine using standard studio musicians and only having to make record deals with the “star” make things run much more smoothly. It would remove much of the band friction that ended acts like The Beatles and Guns and Roses. If the bass player for Alan Jackson can’t or won’t play for whatever reason the studio just replaces him and the tour goes on.
Often by their second album artists can record their own material; like Shania Twain and Jamey Johnson.
I thought the documentary Trashville (starts off in Finnish IIRC, but it switches to all English) was a very interesting take on the big record companies in Nashville and the artists that hate the modern Nashville music industry. Hank Williams III lyrics: “pop country really sucks!”