Asset Forfeiture laws generally only apply to the proceeds from certain felonies, particularly drug-smuggling cases. The forensic accounting investigations necessary to develop enough evidence to seize assets from real estate, bank accounts and other tangible property involved in serious organized crime are lengthy and complicated and generally only worth the effort if major assets are involved. They also do not apply to most illegal immigrants, as they typically do not own real estate, expensive cars or maintain large accessible bank accounts, and there is typically no statuatory authority for state or local officials to seize those assets, even if they existed.
For example, most forfeitures of cash from drug dealers typically do not start until you hit about the 10K mark, because the expense of conducting the investigations, developing evidence to support that the money was invloved in drug transactions, and legal bills (even .gov lawyers cost money) make it "not worth the effort." Despite the typical Internet myth that LE can seize cash simply and routinely, it actually does take a fair amount of investigative work, proof of criminal activity and some solid evidence before LE can take money, cars or other assets from suspected criminals. There is not authority for LE to do so when the only offense is illegal entry into the US.
I will add that many illegal immigrants dealy primarily in cash (they do not trust banks and have difficulty opening accounts) and it is routine to arrest these folks for more run-of-the-mill offenses like Public Intoxication on a Friday night and find that they are carrying a couple grand in cash because they just got paid.