It's incredibly difficult to contain. These are very remote settlements and deep inland. Furthermore the government is ill-equipped to fight against it all. To have been working for years now on a project to protect the Amazon rainforest in southern Peru, even when pointing the illegal farming operations the government can't do much. At best they are overwhelmed, more often then not they don't even bother intervening if it's not on a national park land or border. Only private conservation efforts are managing to help slow down the spread, and they are very very localized.