Steganography is usually meant to refer to covert, deniable information encoding, while extra data is too unsubtle to be hidden from an adversary. It's a simple and convenient trick to hide data from unsuspecting users who don't look for secrets, a weaker threat model.

Actual image-based steganography schemes, as used e.g. for watermarks, usually alter pixels in a way that is approximately (and hopefully) undistinguishable from plausible noise.