I don't think we should overthink stuff, but (SPOILER WARNING) the Eternals purpose isn't only to protect the world, but to make its civilization grow, and the current status of the planet/human population is supposed to be the state of maturity at which the Celestial can pop out. So there's no contradiction in having a group of different looking Eternals that will work on the earth population and end up with a situation that mirrors their own appearance. Either it evolved naturally that way but it was planned as "that's how you breed Celestials", or it may even be a consequence of the Eternal contribution (it's not the Eternal who looks like the people it's watching, it's the people who look like their Eternal).