All of them can be created or modified artificially. Interesting that we can not only tolerate but even enjoy if we change the last three (using compressors, filters, reverbs). It’s usual and everybody likes it.
But the most important ones, the first two parameters are different from this point of view. We don’t want them to be artificially changed – or more precisely: we don’t want to know about it – because then we consider the result to be unnatural/fake.
This is true for all live players but we are particularly sensitive to human voice. Artificial voice (including auto-tune, vocoder, talk-box, computer-generated singing) is a part of certain genres where it is clear for the audience because everybody can see this or it is easily audible.
So, I think the problem is, when a live performer is using these corrections on stage inconspicuously and without informing the audience about it, pretending he/she can sing or play so good without anything. This is a lie, worse than playback.
(It's like paying for a concert ticket with fake money.
