Why would I need to be lucky? When would I care how many ounces of protein are in my sandwich?

serving size is usually in imperial

All packaged foods in the US are required by law to be dual labelled in metric and customary units since 1994. The package size, serving size, and nutrition facts are all labelled in metric.

It is really a myth to say that the US "doesn't use metric". (and it's also a myth to say that they use imperial units, US customary units diverge in many cases from imperial units) The US widely uses a mix of both customary and metric units.

Don't worry, our labels say calories, but are really kilocalories.

The serving sizes are often in imperial (although the closest label shows serving size 1 can). The quantities of fat, sodium, sugar, protein are all grams or milligrams for sodium. But ounces and grams for those things are equally unrelatable for me. I don't consider the mass of these items, I just know them as a quantity on a label. If I were cooking and adding sugar or salt, it would be by volume, not mass.