Why customers pay tips based on the percentage of the total bill even though it might not be reflecting the actual service received?

Let's say customers in table A order two lobster pasta- with giant lobster, and a bottle of wine. 150 dollars after tax. two plates of food.

Customers in table B order 10 plates of tapas, tap water. 80 dollars in total after tax.

Now come to the number of tips, usually people pay 20%, so customers of table A pay 30 bucks for tips and customers of table B pay 16 bucks. Apparently, table B receives more service, but since the total bill is less, the number of tips is less. Isn't this not fair for both the customer and the server?

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Thoughts?