Because cars, homes, watches and jewerly don't jump in price as highly as art pieces do. You can't buy some shitty car and sell it for 1 million a few years later. As for selling antique cars and Ferraris and such, those are already expensive to begin with and the margins are low.

But you can artificially inflate some BS artist's work.

These things generally aren't unique, and so it's relatively easy to establish a market price, and flag it if something suddenly sells for a lot more than that.