Completely agree with
@Foxi. In fact, I would guess OU rifle production has outpaced the SxS configuration by at least ten fold over the last century. Had Germany not lost the First World War and retained its colonies in East and Southwest Africa (modern day Namibia and Tanzania) I am certain OU rifles and metric calibers for DG would be as popular as the English concept of a DG rifle. However, they did, and when wealthy American sportsmen began to arrive in numbers in the twenties for their first African Safaris it was to be greeted by someone with an English accent carrying a SxS rifle built in Birmingham or London.
I know this is heresy, but the SxS configuration is no faster to reload (prove it to yourself with an OU and SxS shotgun), is more difficult to regulate, and is far more difficult to scope effectively. However, since the 20’s the “double rifle” became synonymous with SxS with those who could afford them. When the Renaissance in traditional DG calibers began in the late eighties it was only natural that the American ammunition company leading that charge produced ammunition for those traditional rifles.
Meanwhile, on the Continent, German and Austrian hunters continued their love affair with the OU rifle. Because these rifles were being used primarily in Europe, the 9.3x74R dominated as the “heavy” caliber, and those rifles are built and regularly used today. They are particularly popular for driven wild boar.
So, there is no mechanical reason that the OU isn’t more often built in traditional DG calibers (though a quality .375 is not difficult to find). If you don’t need to ask the price, a call to Hartmann & Weiss will get you one started in any caliber you like today.
If those first Americans had been greeted by a “white hunter“ named Hans, I suspect our choices in action and caliber would be far