Because the artist doesn't necessarily know what's going on while mixing.
Autotune is all over records nowadays and many a time the listeners absolutely won't notice because when they think autotune, they think about Cher or T-Pain bullshit.
If used sensibly, it's a great tool to fix minor intonation issues without these infamous artefacts. It's basically absolute standard to finetune doubled/harmonized lines, which sometimes sound like crap even if they're just a few cents off. So even highly talented singers can benefit greatly from that technology in times when recording budgets crumble.
But it also get's frequent use on bass and even sometimes guitar (DIs).
Sneap, compared to some other mix engineers who are highly en vogue right now, resorts to much less "trickery" or "fakery" than many of you probably think. He's just an enormously skilled engineer who has a very strong vision of what he expects the results to sound like. If there's lots and of tuning, reamping and sample-replacing going on on a Sneap record, it often means he didn't track the stuff himself...
I, personally, like his very distinct style as much as what NK did with them, but it's still up to the band in the end to deliver the songs making it a great or not so great record.
And like him or not and like this fact or not, but you simply can't deny Sneap having a significant role in Nevermore's heavy rise in popularity ever since Daad Heart was released.