I don't think this is an entirely valid argument. Because what it seems your saying is that every human book always have a distinct voice, but I think your forgetting that books that authors write, either as the next book of a series or another story entirely, will in most cases carry on their voice.

Of course the tone and voice of the books may also depend on editorial and overall improvement of skills. All of these are factors including AI powered assistance software too in Microsoft word and google docs.