Because they get paid by the hour; if it takes longer to make your drink, they already get paid more.
Would you tip the kid at McDonalds when you order a burger "hold this" or "extra that"?
It's your buck; tip 'em if ya want.
I won't.
I see a big difference between factory food (McD) and real made to order. Also keep in mind in the factories there’s a lot more division of labor, the guy at the counter did NOT make your food, a guy in the back did, maybe your guy up front dropped the fries in the vat, but in this self serve drink world they didn’t even get that for you. The barista probably took AND made your order. They devoted significantly more time specifically to you than the entire store does.
Then there’s the real question of if you don’t think this person is worth one extra measly dollar why are you letting them make stuff you’ll put in your body? Be generous, it’s good for the soul. And remember the more you tip the more they work for you instead of the company.