Although we are exquisitely attuned to find differences in faces, there are only so many types of structures that make a normal human face, and with billions of people in the world, it's not surprising that unrelated individuals end up looking alike simply by chance. We underestimate the role of chance in life, and that's why we are always surprised by what seem to be impossible coincidences, but when we actually calculate the probability of these "oddities" happening, they are not that odd. It's a common cognitive bias (for more on probabilities and cognitive biases, see here: List of cognitive biases).
Also, once somebody starts posting pictures of look-alikes in the internet, we immediately think that there are more cases of look-alikes than there may actually exist, because of availability bias (Availability heuristic). Especially when the people posting the pictures look for (and improve by Photoshop), the pictures that best confirm the resemblance, and ignore others in which they don't look so alike.