I definitely get why people say "oh, I don't see cliques, just groups of people who know each other", "it's just about how you view the world". But there is definitely a mentality or dynamic that comes with certain groups of people. To pretend there isn't would be disingenuous, I think.
As someone who usually keeps to the periphery or these groups, I do find it interesting, but not always pleasant, watching it from the outside.
The slim friend telling her vastly overweight friend how worried she is that the overweight friend is losing too much weight. The friend telling a nice story about her twins getting along brilliantly only for another friend to snortily interrupt, saying how amazing her child is too you know. Jesus, it looks horrific.
I once accidentally set in amongst a big group of friends at playgroup and the passive aggressive comments some of them started making that I was taking Sarah's seat, honestly made me inwardly snigger, before I smiled and loudly said, "oh look, there's another seat, so I'll move there and you can all sit together" and moved to sit beside a friendly lady who was on her own.
I can imagine if I was someone who desperately wanted to be part of the gang, that might have been a bit hurtful. As it was, I found it quite funny.
I have moved around A LOT in my lifetime, especially as an adult. Where I now live, the 'cliques' tend to be people who have lived in the area a long time and know each other from school. On one other hand, I have heard that if you take out a bottle of baby formula in South London, you'll probably get the cold shoulder, but they are more accepting of 'blow ins'.
Do, your experience with cliques, probably depends a bit on where you live, whether you are from there originally etc more than just your world view 
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