I guarantee you that people trying to raise a family on a budget pay very close attention to the cost of individual items.

People might stop caring quite as much about individual pricing once they have enough money not to have to care.

People who have to worry "can I afford all of this once I get to the till" are very aware of how much everything costs, and they are more common than people who don't have to worry.

A pint? I mean no, but that's because milk is sold by the gallon. If you think people aren't acutely aware of the price of a gallon of milk then you live in a bubble of wealthy people. If milk rises above $3 a gallon or falls below $1.50 half of my Facebook feed has something to say about it. Same goes for many other standard products.

This thread title starts with "UK:", where milk is sold in pence per liter (and also "metric pints", which are pint-multiple containers quoted in mililiters)

Outside of chemistry, liquids are typically measured in litres (AmE: liters), a metric unit of capacity equal to the volume of one kilogram of water at 4°C (39°F) and at standard atmospheric pressure of 760 millimeters of mercury.

"Gallons" are not a universal metric:

For the US, 1 Liter of milk = 0.2641720524 (US) Gallons.

For the UK, 1 Liter of milk = 0.2199692483 (UK) Gallons.

Because this has been and still is confusing, the standard unit of liter was introduced as a universal alternative (no flamebait intended).

I work in a supermarket and customers are definitely aware of the price of milk, butter etc down to the cent.

If you have plenty of money to spend, sure.

If you are poor and/or live paycheck to paycheck (for example due to your monthly home heating bills having gone from 400 euros to 3000 euros year over year), I can guarantee you, you pay very close attention to how much food costs.