Because they are using her likeness without her permission. If they just wanted a “similar” voice they could have hired one from the start. They wanted
her voice and when she said “no” they used it anyway.
The “her” tweet makes it exceptionally clear that it was always intended to be Johanssen’s likeness, not some other voice actor’s that happens to sound similar; OpenAI is just betting they can get away with it.
It seems like OpenAI were also betting on Johanssen not knowing her rights _de jure_in that area, which it appears they were quite wrong about. (And even if she didn’t, she would have quickly learned of them in this hyper-connected world, making the bet utterly incompetent…)