Denmark is a very small country and what works there doesn’t necessarily work at the scale of the US or China.

Would it work in Wisconsin, Minnesota, South Carolina, Alabama, Louisiana, Kentucky, Oregon, Oklahoma, Connecticut, Utah, Nevada, Iowa, Puerto Rico, Arkensas, Kansas, Mississippi, New Mexico, Idaho, Nebraska, West Virginia, Hawaii, New Hampshire, Maine, Montana, Rhode Island, Delaware, South Dakota, North Dakota, Alaska, DC, Vermont or Wyoming?

They all have a lower population than Denmark.

All but six states (CA, TX, FL, NY, PA, IL) have less than double the population of Denmark.

Top US universities actually have higher teacher/student ratios than top EU schools, and I wouldn't be surprised if this pattern holds in general, so it would be easier to do in the US than it is here.

Piling on. Why? Are class sizes that different? US and China have many more professors too, is it not proportional?

The US consists of many small parts too, and it may work in each of those small parts.

The important variable is the staff/student ratio, not the size of the country. With few students per class, or many teachers, this scales fine. I'm running similar exams for my (UK) Masters modules, but colleagues are reluctant to do it for the enormous compulsory survey courses as there are several hundred students in them (vs up to about 50 in the MA ones).

Why not? This is such a thought terminating statement. No actual reason.