Why Americans take their shirt off when running?

No you don't. You'd be using the metric system if you truly believe in science, not the cave men system.

You don't protect anyone but your self interest. Its a capitalistic nation that doesn't do anything without getting something in return for its investment. Your militaries are in Europe (and everywhere else) to assert dominance and control. Nobody wants you anywhere and it's not a charitable act. I roll my eyes everytime I hear someone say "America protects the world". Educated Americans don't actually believe that, only ones who are into propaganda news.

I’m Australian and we run not track sessions shirtless - for the same reason as others in this threads: bc it gets really hot. I always feel funny about doing my easy runs shirtless though, because I do them on streets

Why do the British think Americans care what they believe. True Americans don’t care what the rest of the world thinks.

I'm 68 now so I would never run without a shirt these days, but back in my competitive years, 1980-83, I trained without a shirt all the time in the summer. I live in the south where it can get very hot and humid and I'm a heavy sweater. Back in those days, all we had were cotton t-shirts, which were very hot and heavy when they got wet. Running shirtless just made more sense.

It's not just Americans - a ton of Aussie runners will whip their shirts off as soon as the weather turns even remotely mild.

Because Europe is better than America. There. You happy?

Why is this even a topic for conversation? If you go to a busy beach during the summer you have people of every age - young kids through to great grandmothers dressed scantily and people just accept it and don't bat an eyelid. A guy running down the street on a hot summer day shouldn't be any different.

FINALLY someone noticed our outcry! Having our skinny American high schoolers run shirtless was our passive aggressive way of complaining to our European friends about what they have been doing to us for decades. That’s right. Now that you have noticed, it is time to make a deal. Our skinny American high schoolers will stop running shirtless and your hairy fat men will stop wearing thong bikinis at pools and beaches. We think that is a reasonable trade. We are sorry that it had to come to this but they wouldn’t allow it in the Paris climate accord.

I do it to make fat people jealous

Then can we get our tens of billions back from Ukraine? How do I request that from Sleepy Joe?

Ptetty sure the Ingebrigtsens do it all the time in their reality show.

There are things called paragraphs.

I quit caring what Europe thinks in 1776.

There's a very simple answer:
It is generally hotter and more humid in the US. The average latitude of the contiguous US is about 15 degrees below the avg Europe latitude.
Examples:
Florida = Sahara
North Carolina = Morocco
Maine = France and Germany
Canada = England

IDK, I've seen a lot of people in the UK do this as well.

Let us look at the example set by the most successful European runner of the 21st century:

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Maybe the OP is a racist who only disapproves of white people running shirtless?

Florida. High of 80 deg F in my town today in December. Lows are still in the mid 60s.

I run near the beach where a lot of unfit people are shirtless as well. My shirt stays off unless it’s an easy short run or gets below 65 and low dew point.

Ditto, I do this in Germany (infrequently, but still) and see plenty of others do it too. A couple caveats:

1) I've only seen people do this on dedicated running trails / parks where you can run uninterrupted for miles and miles. You do not see people running around shirtless in the street EVER.

2) People don't walk out of their flats shirtless. They wear a shirt to the park then take it off when it gets hot. Maybe this is a commuter thing since most people don't drive to the park in primarily urban areas.

3) Boardwalk / beachfront area (think French Riviera): Forget it--half the runners are shirtless / in sports bras, including locals.

Side note: I actually see a lot more women shirtless in sports bras than shirtless guys, though I still see both.

It's's also not unusual on a sunny summer day to see non-runners spontaneously strip down to underwear / swimsuits and lay out or lounge around on park benches. Gotta get that Vitamin D when you can.