“They’re all the Same!”...But for Several Different Reasons

One reason for perceived group homogeneity is that it reflects social reality: People may perceive group members as being "all the same" because they are, actually, very similar to one another, as can be seen in this group of the Queen’s Guards!

Another reason is that people, especially Westerners, are motivated to perceive people in their own groups (ingroups) as a bunch of individuals and people in other groups (outgroups) as being "all the same". This perceived in-group heterogeneity lets people express their individuality and distance themselves from any negative aspects associated with their group.

It also matters what kind of group is being judged. There is a tendency to judge people in small groups, low status groups, and low power groups as being relatively similar to one another.

Finally, it matters what dimensions are being used to judge the groups. Group members tend to be rated as being similar to one another when they are judged on stereotypical dimensions as opposed to nonstereotypical or counterstereotypical dimensions (e.g., men judged on the dimension “adventurous” rather than "sensitive").

So, whether or not “they all look the same” depends on their actual variability, whether you are one of them, how numerous and powerful they are, and what kind of dimensions you’re judging them on.