[Strangler Lewis]

Because the amounts and the atmosphere differ. At a potluck, each person or familiy's contribution will reasonably fall between about $5 to $20. People mill about, and nobody pays too much attention to what anyone else eats or drinks (unless they get drunk). At a restaurant, every time the server comes by to take an order, a little cash register can go off in everybody's head. Drinks, more drinks, appetizers, entrees, dessert. It adds up.

MP, I would also bet that when you go out for dinner with friends, there's a lot of signaling going on about who's having what, splitting appetizers, dessert/no dessert, etc., so that at the end of the day, splitting the bill seems within the realm of reasonableness.