Because shops and retail parts stores mark up the parts so high, especially on cats! Also, it depends on how the exhaust was designed on your particular car. Some cars have the cat along with a long pipe, and some have it built into the manifold; both of those designs cost a lot more.
If you bought it on your own and took it to the shop for them to install, you could've saved a lot of money.
Online, like on Rock Auto, a converter with a whole pipe attached, or one with an integrated manifold, can go for $300-400. A converter that has neither of those attached, and is just a cat with a flange attached on each end, is usually under $200, sometimes as low as $100. Then there are universal cats that have to be welded on, which cost even less.
The precious metals used are expensive, but that is only part of the story.
Federal-spec cats are required to have a 2-year/25k warranty on catalytic performance and a 5-year/50k warranty on structural integrity. California requires CARB-approved cats to have a 5-year/50k warranty on catalytic performance.