When you arrest this person, what happens to their assets that they've (assumingly) illegally obtained? The hundred thousand in drug money? The guns, high end electronics, luxury cars, etc?
You weaken the individual criminal and their gangs by taking their money away. It's far to easy to just pass those assets on to another person and they can simply assume the role. And after the person does their 5-10 years in prison should they be released back to what they had obtained through illegal activity?