Even cash is slightly more "marked", since bills have serial numbers, and it's more difficult to launder large amounts of cash than to send BTC through a mixer. Doesn't help much in small-time cases like having $100 stolen from your wallet. But it makes it harder to make off with a large amount of cash and then actually spend it. A common way of tracking bank thieves is to blacklist the serial numbers and wait for alerts from cash-counting machines that check the blacklist (mostly at banks and back offices of large businesses). Then investigate the areas where blacklisted serial numbers pop up. It's not foolproof, but it makes it much harder to successfully do anything with $10m in stolen cash.