Because our country is very narrow. Running across the width of England is only a few hundred miles as the widest part. Not nearly grand enough for Rob.
However, he did run his 'longest run without sleep' in the UK, I think. So I guess Brits sometimes do run their fake runs across England.
Then why not run across Russia? Or Australia? Why come to the US and destroy our productivity by causing us all to waste countless hours laughing and debunking the run?