> Why is catalytic converter theft seemingly worse in the US/UK than in Europe/AU/NZ?
Many reasons, but chief among them are that EU is oddly really relaxed on emissions (hence diesel gate not being as big of an issue in it's own Country) and don't use the same equipment on their cars also because AU/NZ are big markets for used Japanese export vehicles that have exceeded its Shakin/Km limits in Japan and there isn't the local demand for these components as the laws don't reflect such strict standards.
By contrast those cars are not legal in the US due to emissions, until they fall out of the testing period: see used the massive headache created with the post Motorex Nissan Skyline market by-laws. The UK is odd because MOT and carbon tax things play a part, but it's way easier to register a RHD car from Japan if you're outside of London, and even within it if you know where to go. But both the US and UK also have lots of poverty based crime (theft) underlying their society.
Europe, but especially Germany, has many auto-manufactures writing their laws for them; and the higher ubiquity of diesel cars on the road in those markets as opposed to benzene (gasoline) means they cannot just make them all vanish without having immense backlash. So, they greese the right wheels, and things carry on: business as usual.
I won't touch the other reasons, but if you really want the nitty-gritty of how relaxed EU standards are read: Faster, Higher, Farther.
There is a reason it was some college kids in CA with a home-brew emission tester in a rented Tdi that butsted VW, and not the EU's climate Nazis/Green party in Baden-Wuttenburg. (I lived there and was subject to their stupid vanity based green washing laws as an environmental activist, so I can say that!)