One of several reasons.
My primary reason is because THATS THE WAY IT FUCKIN' IS. You go to a restaurant, this is our custom. You also don't shit on the floor, or play with your dick at the table, or shove people out of your way into the fuckin' lobster tank.
I hate this fucking argument, from snot-nosed little twerps who think they've had an original thought. Whee!!! I have a contract with the owner, SHUT THE FUCK UP!
Here is a simple and straightforward "contract" for ya. If you don't have an extra fifteen percent beyond whatever you expect to pay for the food, don't even go inside. You ACCEPT the possibility of gratuity by simply walking in the door. The owner EXPECTS you to understand the customs of the land, or to stay the fuck out.
Now, if something unexpected happens, you may alter the gratuity. Less, or more, depending upon the incidentals. But if everything is as it should be, then they have fulfilled their half of the "contract". Which means YOUR refusal is the breach.
Idiot.