"Humans are adaptable creatures" makes sense. It means that when things change, we can change with them. "Humans are habitable creatures" is almost nonsense. As Marjon says, the only possible meaning is that we can be a habitat for something else--fleas, lice, parasites, bacteria. However, even a biologist would not use "habitable" that way. "Humans are creatures of habit" make sense but means something different from "adaptable." It means that we tend to establish habits and repeat the same patterns of behavior over and over. There is a word "habitual" but I can't think of a way to use it hear. "Habitual" means "the things that are a habit." Coffee is my habitual morning beverage. It is my habit to drink coffee every morning.

Humans are adaptable creatures. They adapt to anything. 'habitable' would mean, humans can be lived in (by bacteria I suppose ;->)