The "custom" of tipping in the U.S. is so well established that Congress has presumed the fact of tipping in our Minimum Wage laws. Those who work in tipped jobs have a much lower legal MW, on the assumption that tips will supplement the actual wage to make it as high or higher than the MW for everyone else. Competent servers make much more than MW, and would rue the day that tipping was outlawed and their employers were required to pay them $7.35 or whatever the hell it is now. They would lose, big time.
Tipping is NOT universal. In the EuroZone table servers are almost all MEN, and they are paid as well as some skilled workers, which is what they are. Menu prices are accordingly much higher than in the U.S. Because of this, a couple going out to dinner can expect to pay a hundred Euro or more for dinner in a "nice" restaurant, with maybe a single glass of wine (EVERY glass of water or soft drink is charged anew - no free refills). If the service is extraordinary, the clients might leave a Euro on the table to acknowledge that fact. Given the exorbitant price of restaurant food in, say, Germany or Italy, a 20% tip would be highway robbery.
In these days of the Cajuna Virus, the "standard" tip in the U.S. has informally been increased from 1/7 or 15% to 20%, recognizing that servers will not have as many settings in the course of a week under Pandemic conditions. If you do not pay that for competent service, you are a dick. If you can't afford that, go to a fast food place or eat at home.
My personal pet peeve is "tip jars" in places where the employees are paid MW or better. Bullshit, I put nothing in the tip jar unless the service is EXTRAORDINARY.