Because it's not authentic, it's "acting authentic".

There's a 1987 movie called Broadcast News which touches on the issues of how reporters portray the news and what can happen, and why it might be considered unethical to add touches and flourishes that add authenticity to inauthentic events.

NPR Voice is attractive, I enjoy listening to it, but what else can they sell me while they use that voice?

If NPR Voice is not spontaneous but heavily practiced, rehearsed and edited, how is it different from propaganda?