If you had shit on your hands, would you consider them clean after wiping them with a paper towel a few times, or would you want to wash them with water? The same applies for your butt.
Different regions of the body, and different uses. Around the anus there's bacteria in a nice warm moist environment to go to town. I don't open doors with my asshole, so I'm happy with enough wipes to get any excess poop off, followed by a spray from a cheap cold-water bidet (or diaper sprayer, left over from when our child was in cloth diapers), followed by a wipe for drying.