Perhaps laziness? Determining which profits are due to a crime could be difficult to separate from legitimate activity. That said, they could write the law to claw back anything that's easily accounted for, and just use some approximations when that's impractical.
It doesn't even need to be an exact or accurate accounting of money made due to crime. The legal system could impose fines high enough to hurt regardless of where the money came from, and pretty much everyone agrees that they should because otherwise fines are just a cost of doing business. The fact that the legal system refuses to do that when it's so obvious that not doing it is harmful to everyone except criminal corporations is troubling.
