To the contrary, I've seen 3.8L or 3.78L written on every gallon jug for my entire lifetime. I know that conversion off the top of my head... even before I had mandatory science classes where I formally learned the same.
But even if you haven't read any of the text on a milk jug over the past half-century, surely you've seen one of these before?:
They said the same thing here in the UK years ago (the 90s I think) when they started forcing labels on food to have metric.
"I want a pound of sugar, not 453g!"
Turns out the world didn't actually burn down. The containers mostly stayed the same size, but I think they slowly gravitated towards logical sizes (so e.g. that 453g package became 500g or 400g at some point)
The FPLA requires metric units to be included on labeling.
https://en.wikipedia.org/wiki/Fair_Packaging_and_Labeling_Ac...
https://www.nist.gov/standardsgov/compliance-faqs-packaging-...