There are at least two major difference between civil asset forfeiture and civil litigation. First, the plaintiff is the government, and the government has vastly more powers and resources than typical plaintiffs, making it more difficult for ordinary people to defend against them and more reasonable to demand from them a higher standard of proof. Second, in typical civil litigation, the plaintiffs don't seize your house/car/money first and put the burden on you to initiate litigation to get it all back now that you no longer have any resources with which to fight that battle.