Because the most obvious thing for OpenAI do would be training the voice on Scarlett Johansson's available body of work, from movies, audiobooks, podcasts etc. Why bother hiring a voice actor?
Anyone can come up with DL networks these days.
https://www.reddit.com/r/singularity/comments/1cx1np4/voice_...
This whole controversy makes no sense and is obvious that the media is looking for something to attack openai with.
Apparently they selected the actress from auditions in early 2023, a while before the Johansson timeline.
Of course the OpenAI folk would likely have seen Her and may have selected someone sounding like that.
If they deliberately used an actress that looks like her in an advert. A bit of makeup, a latex mask, digital touch up, full deep fake; at some point most people can no longer tell the difference.