The older Honda Jazz was notorious for cat theft. IIRC it was just that it was very accessible.

Can someone explain why it’s mostly Japanese cars that are targeted for these thefts?

It used to be any cars with sufficient ground clearance to make the removal easy but lately I have seen Honda Jazz, and Toyota Auris losing their cats.
The availability of cheap disc cutters may go some way to explain but sometimes the cats are unbiolted.

Perhaps the Japanese cars are more likely to be found in third world countries ?

But when a farmer friend who was hosting a car boot sale was told that some of the cars in the car park had reported the theft, he immediately locked the entrance to prevent the van that was being used to store the stolen cats leaving .

He called the police to tell them he had the criminals together with the evidence trapped on his site they declined to attend and offered crime reference numbers for the victims to give to their insurance companies.

It tends to be cars with insufficient space in the engine compartment to allow positioning of the catalytic converter next to the cylinder head so it has to be fitted down stream under the floor - which makes it accessible to thieves.

The converters aren't used as spares, they're processed for their precious metals inside.

But when a farmer friend who was hosting a car boot sale was told that some of the cars in the car park had reported the theft, he immediately locked the entrance to prevent the van that was being used to store the stolen cats leaving .

Perhaps he should have locked the exit .... :-))