Why ? Should good service not be the standard, rather than something unexpected, worthy of special notice ?

Anyway, the "underlying notion" fundamentally has little to do with good service, it's pure economics - waitstaff get paid less than minimum wage and the assumption is that a significant proportion of their income will be made up from tipping (most importantly, the Government assumes this when it taxes them). The idea it's a tradition to reward "good service" is exposed as specious simply by considering all the other service-based jobs where tips are not given. Do you tip your accountant, your lawyer, or your dentist ?