This isn't a uniquely American thing to be fair, in the UK it's not unusual to discuss area in terms of football pitches or for very large areas, Wales.
To be fair, who actually has a mental model of what 20,779 square kilometres or 8,023 square miles (the area of Wales expressed as a number) actually looks like? Doesn't matter if you're using imperial or metric, it's not very intuitive beyond the human scale.
If you're trying to communicate 'this much rainforest disappeared' or 'look how large this landmass-sized Antarctic iceberg is' to a wide audience a well-known area is a reasonable way to do it. In Britain basically everyone knows roughly what the size of Wales is relative to the other countries of the UK, even if they don't know its exact size.
20,779 square kilometres you say? Thats trivial, its roughly strip of 20km that is 1000km long. Or 200x100km. If you can't imagine that, thats mostly problem of your practical imagination, I have the image of such area in my head immediately and can map it into various parts of regions around me rather easily.
I don't play football, nor soccer nor rugby nor baseball or similar sports, just like you most probably don't climb on rock. I am not pushing my metric of '2 spans of typical rope length for alpine tightrope 2-people setup ' or similar obscure metric, do I.
What is good about metric is that once you have a good feel for different lengths of mm, cm,meters and kilometres you don’t need to worry about things like “10 Super Bowl stadiums”. You can easily extrapolate what 27km2 means, it doesn’t have to “look like” anything.
That really isn't true. You in particular may be good at such extrapolation, but people as a rule are not. They need a concrete example to relate back to.
The UK is arguably worse than the US in this regard. The US has mainly stayed with a system that sticks to one measurement across use cases. If I burn fuel at a rate of x miles per y gallons, I purchase it at the pump in gallons. If Tommy Atkins burns fuel at a rate of x miles per y gallon, he purchases it at the pump in liters; there's an extra step for conversion.
