As mentioned, Dakota sells them and for a lot less than $45K. Nevertheless, most are probably made up like mine by rebarreling a .375 or .300.

How many have been made in 15 years? I don't know. But how fast a cartridge catches on isn't always related to it's usefulness. We have had some flash in the pan calibers that sold like hot cakes but all but disappeared in a few years. We have had some slow starters as well. And .400 H&H rifles are being made. They are being made by H&H, by Dakota, and by gunsmiths whose customers occasionally order one made up.

The .416 Rigby hasn't been made in huge numbers either. There have only been about 2000 .416 Rigby rifles produced - ever. Ruger made over 1000 of those. And in the entire life of the cartridge, 105 years, Rigby has only made about 550 of them. It was Ruger who brought the .416 Rigby cartridge out from the fringes with a mass produced rifle and it took eight decades for that to happen. And only then were ammunition makers willing to take it seriously enough to add it to their catalogs. Before that, options for loaded ammo were very limited.

Remington's .416 was and is successful in no small part because it is Remington's policy to manufacture ammunition for any rifle they produce and Remington is one of the largest sporting ammunition producers in the world.

The single biggest thing holding the .400 H&H back in the USA is the availability of mainstream ammo. I am confident that will eventually change and that it will take less than 80 years to do so. For now, it is a matter of special order or load your own. But .400 H&H head stamped brass is easy to find and costs less than .416 Rigby brass. And .410/.411 bullets are aplenty.