Not exactly true. Carrie Underwood, Rascal Flatts, Blake Shelton, and others are regular members of the Opry, and see it as an honor to play there.
And, to a considerable extent, being invited to join the Opry is a sign of "arriving" to newer artists. It's a way of saying, "How do you like me now, now that I'm on my way?" to the world.