Because the person serving a beer at a bar simply poured me a beer. I walked up to the bar where I ordered from the menu. The level of interaction with staff = 1. Effort from staff = 1.

The the waiter/waitress had to come to my table, discuss the menu, provide feedback on questions, submitted order to kitchen, delivered order, checked back on us to see if we need anything else, etc. Level of interaction with staff > 1, level of effort from staff > 1. Tip appropriately if level of effort > 1 for helpfulness, politeness, attentiveness, etc. Stop making this so hard.