I've been confusing: my apologies.
First, @ofwarren : no, I don't tip delivery drivers like Amazon. But I do tip for takeaway food delivery, like Deliveroo in the UK or GrubHub in the US.
Second, I deeply regret making the house comment, @Butterflyfluff, because now I'm going to have to explain it and look, as I said, like an absolute dickhead: no, of course I don't tip my hosts if I go to stay somewhere. But if you're invited to a country house for a party and staff looks after you, it's appropriate to tip. It's something my grandmother was very firm on.
As for the person who asked how you reconcile the arguments that you tip in America because the servers need to be paid to the argument that NYC bartenders make pots of money...it depends where? Someone working at a WaffleHouse in rural New Hampshire or a diner in Kansas is probably poor and needs the money. Someone serving at Jean-Georges is probably doing fine.
Anyway, tipping isn't the reason it's expensive to visit America. Yes, you should add 15-20% on top of meals and leave $5-10 on the dresser when you check out of a hotel. But, in comparison, the tax rate on your food will be about 8% rather than the 20% VAT. (Not saying I approve; this is just true.)
It's expensive, at least in the moment, because the pound is absolute shit due to a terrible approach of shock supply-side economics coupled a lack of manufacture due to Brexit that makes the blow impossible to cushion. Oh, and a weird decision to do all this whilst increasing national debt so the population doesn't freeze. That's the reason it's very expensive in America.