Blame the payment processors. If you are a store and you buy a point of sale machine, it comes preconfigured to 18%/20%/25% or whatever tips, and it is not well documented how to change it. Most small business owners give up and stop bothering.
Why do the payment companies do this? Because they get ~1% of the tip.
Why do the payment companies do this? Because they get ~1% of the tip.