In addition to the fact that these villagers have all descended from a group of refugees from a single, fictitious island, there is also a technical reason for the "cultural" similarity of the villagers. The heads and bodies have to be interchangeable so that different head and bodies will go together. For each head and body there are many different views and poses, and all of those take a lot of work to create. Just like the male and female villagers have to be handled separately (so that a male head isn't placed on a female body, for example), each different skin tone would have to be managed separately. It's not impossible, but isn't necessarily a practical thing to do in Virtual Villagers.