This isn't supported by research. Humans are social animals that want to communicate. Successful communication begets more successful communication. More likely is that either there is something preventing this child from developing the speech abilities, or it's developmental and she'll get there when she gets there.
Sign language is successful communication.
But I think geofizz's point is regarding your original claim, that "Talking is hard. Why talk when there's signing? The primary motivation for kids to work on their linguistics is the need to be understood. She doesn't have that need, and so, she's not working on it. It's not a priority."
Kids who are developing normally don't need to "work on" acquiring language. It's what their brains are built to do. Expose them to multiple languages, they'll learn them all. Hearing kids who grow up in deaf signing households learn the spoken language of the surrounding community with no problem.
I agree with others that this is a speech delay, and while it could work itself out (as some here have experienced), it warrants a look from a professional.
