Your last line, I feel, really hits it on the head. People tip, because "that's what we are supposed to do." It's been culturally ingrained in us for decades. So many people who work in the service industry are VERY outspoken when it comes to tipping, and the amount of vitriol and disgust they apply to those who don't partake in the system is frightening. People tip, because it's what they feel the HAVE to do, and because they are scared of being confronted as the boogyman who doesn't tip.
Heck I fit this description perfectly. I despise the system yet I can NOT bring myself to not tip at all. I just do a solid 10% instead of the ridiculous percentage (that seems to get higher and higher every year) that I am "required" to tip.