The precious metals describe why they are valuable, but not why they are stolen.
Consider something else that's easy to steal: bikes. You only need a battery powered angle grinder to steal a bike (and in many cases, the bike isn't locked to a solid object so it can be carried away and the angle grinder operated later; it's also possible to disable a lock with a battery powered hot glue gun, allowing using the angle grinder at a time of a day when nobody is hearing it, since the owner can't unlock the bike and has no choice than to leave it there). Many bikes are even more valuable than catalytic converters.
What's different between bikes and catalytic converters is that if a thief has 100 bikes with no proof of ownership and tries to sell them to a used bike dealer, that dealer probably wouldn't accept the bikes.
However, if a thief has 100 catalytic converters with no proof of owning 100 cars or operating a car repair shop (to explain having 100 cats), and tries to sell them to a scrap metal dealer, in many cases the dealer will accept those cats.
Of course, there are other channels too for bike sales than selling them to a used bike dealer. The thief could sell them directly to customers, advertising the stolen bikes online. That's why we still have bike theft, some people buy cheap used bikes with no proof of ownership, directly from the thief.
What we need to prevent catalytic converter theft is better accounting for used cats. A scrap metal dealer should only accept cats with proof of ownership or proof of operating a car repair business. Used catalytic converters have value only for scrap metal dealers, not for anyone else.
You can also reduce the risk of your cat being stolen. If you engave your registration number into the cat, in a manner so visible that someone considering stealing the cat would see it, the chances of that cat being stolen are reduced.