There is a well known phenomena called "out group homogeneity effect". Basically it means you can't tell the difference (homogeneity = look the same) between two members of another race (out group), even though members of that group can easily tell the difference.
This phenomena happens because the real differences within any group is relatively small, compared to those outside the group. That is WHY they are a group. Chinese people do look a lot more like each other then they look like Swedish people.
But the human mind is flexible - we need to be able to distinguish between people we know. So if we are surrounded by Chinese people as a young child, we learn to distinguish them. The same skill can be learned later as an adult, but it takes a lot more work.
In effect, it is not strange that they all look alike, but it is strange that you can tell the difference between your own group. It is a specialized skill you learned as a child. You see, it's NOT an outgroup homogeneity insensitivity. In reality it is an INGROUP difference sensitivity.
We become hypersensitive to differences within our group. This is something innately human. It is how we learned to tell the difference between poisonous food and very similar non-poisonous food.
Now for the political ramifications. It doesn't just apply to faces. It also applies to politics. We become hypersensitive to relatively minor differences and think they are big.
President Obama is practically identical to Mitt Romney on most issues. As per Political Compass, Ron Paul is the only substantially different presidential candidate. We all know Ron Paul is "out there", but that just means that Barack Obama is NOT out there.
It's not just about about presidential candidates, it's about basic politics. ObamaCare is based on RomneyCare. You don't do that when you have real differences in politics. The Democrats and the GOP are basically very similar groups, with relatively slight differences.
Both sides tend to exaggerate what differences exist. They need to do that to generate support.
But we can easily recognize that they are all pretty much the same, with minor differences.
Note, minor differences DO matter. There is a huge difference between spending no money on Planned Parenthood and spending a reasonable amount. But the real difference is whether we arrest and send women/doctors to jail over abortion - and neither party will let that happen again.