It's a simple matter of fact that human facial recognition is more finely tuned to faces a person commonly sees. Thus, a black person living primarily around black people will think that non-black people look a lot more homogenous than black people do. A white person living primarily around white people will think the same thing. And so on and so on.

So yes, when I look at Asian people, they appear superficially similar to each other, because I'm not typically surrounded by Asian people. Obviously if five random Asian men or women were lined up, I'd be able to tell the difference between them. Duh. But because my brain is not used to immediately differentiating subtle Asian facial features, it's not as intuitive. If I were to live in, say, Kyoto for two years, at the end I'd be a lot better at separating Japanese faces than I was before.

This has nothing to do with prejudice, of course. It affects literally every person, and it's simply a standard behavior of our brains. Our "facial recognition" abilities are really only tuned for a specific species to begin with (I dare you to put ten random pictures of killer whales together and then tell them apart), and since there's a lot of non-trivial facial differences between different ethnic groups, we only "store" the differences of the group members we frequently work with. This can obviously be overcome with immersion into a new group, and it indicates nothing at all about a person's prejudicial tendencies (aka racism) towards any particular group. 

Well, all Asians do share some common physical traits such as straight black hair and epicanthal folds.  Most Asians are physically small sized humans.  I would be a racist if I wrote that all Asians look alike.  We are all members of the human race that evolved from the Mitochondrial Eve and the Y-Chromosome Adam who lived in Africa some few hundred thousand years ago.  Our skin color only matters for protection against sunburns. ... :D

They don't look alike, but in Japan many girls tend to follow the same fashion and make-up trends in order to look like popular celebrities. I think that's partly the reason why some non-Asians find Asians look similar...

Actually , before I learned english and its culture , I cannot tell apart all caucasian people very precisely . It does not matter with racism at all , but rather,  my sole problem is I am not so familiar with different caucasian faces only  .I usually confused some movie stars shown up on the TV channel .To recognize someone's face is not my professional . Since I began to watch English TV more and  dip more  their culture  , I found British and American have different culture in  spite they share the similar appearance . Not everyone who speaks english has similar culture . They are not always sharing the same life style and thought as Hollywood movie showed.

Asian people share the similar traits indeed but that does not mean they are all the same . 

well, I can´t distinguish between people from there, I have heard that it may be offensive if I called a person from Japan chinese or backwards, It is difficult for me to distinguish between people from China, Japan, Korea or Asian countries, In particular I like Asian cultures. I have meet people from China, and they are very nice. Here in my country when a person from Japan or Korea we called Chinese he/she gets angry and immediately answer I am not Chinese, I am Japanese. So I prefer to ask where are you from? before to assume that a person is from a specific country.