The origin of US Customary units is British, even if the US, Liberia and Myanmar are the last countries still using it. The UK has almost entirely adopted metric (yards and miles are still used for measuring distances on roads and pints are still used for milk and beer, and the last government made the eccentric decision to permit pints for wine, which no producer used because they couldn't get the bottles), but these systems of units have identities beyond whether or not they're in use anywhere.

EDIT/CORRECTION: Milk is sold in multiples of 568 mL, so while the quantities are pints, the measurement is metric.

Beer and cider are the only drinks that are legally not sold by metric volume in the UK. They have to be served by the pint, 2/3, 1/2 or 1/3. Every other drink has to use metric.

That's why we call it the US Customary System.

Not really. The UK uses imperial units for most of the things you use units for in daily life (roads, cooking, drink sizes, body weight, utilities, land area...), even though they theoretically converted to metric. Canada is similar.