Clever marketing (shaming) by restaurateurs to shift responsibility for paying their staff from them to the general public.
At one time a reasonable tip was just 10%, usually paid in cash, and went entirely to the server. They gradually increased the "standard" tip to 12%, 15%, 18%, 20%, etc. on the pretext of using the revenue to split tips among all their staff. They also started adding mandatory tips of as much as 20% for groups, then hoped ignorant people would tip on top of that.
Restaurant critics and others dependent on the industry fed into this by also shaming those who left less than 15% (and now even 20%) in tips.
Tipping also got inflated by technology where credit card terminals now offer 15%, 20% or 25% tips, usually on the tax-included amount.
Now with the increase in minimum wage, menu prices will go up so the public pays more for the meal, then even more in tips since the percentage is based on the price of the food/drink. So if menu prices go up by 10% and you continue to tip 20%, you'll be paying an effective tip of 22% on the pre wage increase cost of the meal.
And on and on and on it goes...