They are extremely relatable....I think that may be why.
Also, they exemplify how convoluted and dark the human mind can become when faced with greed. Sad isn't it...how we can so easily lose sight of the things that truly matter. We don't get 1000 years to look back and learn...take advantage of the lessons of Adventure Time. I think the show is highly metaphorical/philosophical and is great for adults as well as children.