Every site will still log your IP if you're using incognito, and Google will still log your searches, even if they don't 'save' to your computer. That doesn't even get into people who use their credit cards to sign up for porn websites, or e-mail addresses that can instantly be linked to a person. There is little anonymity when it comes to searching porn and I think most porn users realize this. They don't care about it because it's not illegal. Sure, they might be embarrassed if a spouse or parent came across their online search history, but it ends there.
Child porn users specifically try to hide their tracks all across the internet - not just by logging into Google Incognito. They do it because they know it's illegal and they can face severe punishments. It's why most child porn users, even to this day, go undetected, sadly - and are allowed to keep their searches. There is a whole seedy underbelly of the internet that most porn users don't even delve into.
So, it's more than just hiding their trail of porn ... it's about hiding their uncovering of it because it absolutely is illegal. Just going to XTube.com is not a crime. Downloading a movie from a porn site of two consenting adults is not a crime (well, beyond copyright BS). However, downloading movies of 12 year olds from file sharing websites, or secret message boards entirely set up to hide any information on the user, is a crime.
Big difference, IMO. Most people who hide their tracks while looking at porn do it because they're embarrassed to get caught - not because they're doing something illegal.