I think the big factor here is that "she" didn't sell her music herself per se. The record label, which got all rights to her music from the contract she signed, decided to sell the rights to her music to another party. She didn't really have any say on the matter if I recall.

The original owners of the music (the label she signed with) seemed to do right by her and she was comfortable with that arrangement, but the new owners of her music have put different demands on her which is severely restricting her from performing her music to her fans.

It is a crazy situation, but happens quite often. Imagine if the owners of the Queen catalog said that their actual music could not be used for the recent hit movie - then it wouldn't have been anywhere near the hit that it was.

That was the problem with the Jimi Hendrix movie a couple of years back. Jimi's sister, who owns the rights to his music now, said they couldn't use his actual songs in the movie, so what you ended up with was Jimi 'sounding' music and it wasn't the same at all...

> On Sunday, June 30 [2019], it was announced that Scooter Braun and his company, Ithaca Holdings LLC, acquired Scott Borchetta’s Big Machine Label Group in a deal that’s reportedly worth over $300 million. Through buying Borchetta’s company, Braun now also has ownership of Taylor Swift’s master recordings made when she was with the label, including her first six albums.

https://www.bustle.com/p/why-doesnt-taylor-swift-own-the-rig...

> in a deal that’s reportedly worth over $300 million

So if she were to really put the money where her mouth is, she could probably buy the rights to her old records with her reported $360 million of net worth, or find a more friendly recording label willing to do so.

She actually tried that, but was unsuccessful.

Artists own the songs (melody + lyrics). Record labels own the recordings. The artist also provides a commitment that they wont re-record their songs with another label for an agreed period of time.