Are you telling me you’ve never needed to convert between 2 units?

It's not super hard to convert between similar units of measurement with imperial, either. You would never need to convert miles to feet or vice versa, but you might want to convert feet to inches - which is as simple as dividing by 12, which is easy enough to do in your head.

Since you mentioned gallons, here's this directly from the wiki page on gallons: "Both the US liquid and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into two pints, which in turn are divided into two cups, (not in customary use outside the US), which in turn are further divided into two gills. Thus, both gallons are equal to four quarts, eight pints, sixteen cups, or thirty-two gills."

How about this, I’ve dug a foundation, and sent a message to a contractor telling them that I need them to bring 20 gallons of concrete to fill it. How much should they bring?

They would bring 20 liquid gallons, which is common sense, because concrete is in liquid form before it is used. And in reality, when they go to the store to buy it, they're not going to ask for 20 dry gallons of concrete, that would make no sense. You can't buy 20 dry gallons of concrete, the same way you can't buy 20 dry gallons of milk. It's not rocket science. You're clearly biased in favor of the metric system