Friendliness or a general level of service is in a lot of industries considered a competitive factor that contributes to increased customer lifetime value through repeat sales, loyalty etc.
Why should the restaurants be any different? If I like the food and the service, I'll want to come again - the restaurant increases their earnings that way and both I and the business are happy.
The question then again comes down to how is that an incentive for the workers. The obvious answer is that their pay should be linked to the earnings of the business. The fact that it is not shows the fundamental problem with current state of capitalism, where the primary goal is to exploit the work of the workers below your financial level (as a business owner) and increase your wealth based on the ever-increasing difference between costs / wages and revenue.
The incentive of business is in conflict with incentive of workers, and the interest of customers - business will be motivated to use the workers willing to work for less, count tips towards salaries, but also use lower quality/cost ingredients etc.
A system where the worker salaries would be linked to revenue, with some nuance of course, would ensure top quality / service / products for the customers, fair reward and aligned incentives for the workers, and access to top tier employees (since they would want to work for businesses with higher revenue), happy customers and increased revenue to the business.