Animals are often healthy when euthanized

It is so sad that animals are often healthy when euthanized. ( http://www.paws.org/about/mag/issues/issue51/livessaved.php) Animals give so much to so many people. They add such joy and have such unconditional love. Please consider all the healthy animals that are killed everyday due to overpopulation. We can put men on the moon. Why can we not prevent overpopulation of animals ? Is it such a dilemma that humans cannot figure out? Is it negligence as a human race? Are there too few who really care?