How did tipping a percentage of your bill come to be?

I hate tipping in general (that’s why I’m here obviously) but what I’ve never understood is why it’s expected that we tip a certain percentage?

It takes no more effort to carry a $10 salad to the table as it does to carry a $70 steak to the table so why should the tip on the steak be expected to be 7x as much as the tip on the salad? I’ve really never understood this logic.