[tarsinge]

> makes the voice sound cleaner, thicker, more polished

I think here you may be confusing Auto-tune with "modern" voice processing, which while happened together are quite different. Modern pop singers are overdubbed ridiculously with added distorsion, resulting in a "massive" (and slightly "robotic") lead voice.

Edit: You can clearly hear this "saturation" in the song Hello by Adele (https://www.youtube.com/watch?v=YQHsXMglC9A), especially in the chorus. Compare with her natural voice: https://www.youtube.com/watch?v=-yL7VP4-kP4