I don't know how or what century it got started, but that is the custom here. We do it because to go against the custom is seen as extremely rude, even ethically wrong. It's like you have customs that have to do with royalty, such as don't stand in front of the Queen. We don't know all those rules, or why you have each particular one.
Failure to tip is regarded here as kind of cheating the wait staff. Whether you tip or not, they will be required to pay taxes on what the tip "should have been," as well as a certain percentage to other staff in the kitchen and those who clear the tables. That combined with low wages means that an actual tip of zero percent can very well result in it actually costing your server a small amount of money per hour for the privilege of having served you.