I'm doing a degree at the moment, and I know a lot of the people on my course would describe our course as a bit cliquey- that is, we have our circles of friends made in first year, and while we are all polite to each other, we don't make that much effort to socialise outside our groups except on big nights like after the last exam.
However, I would say that we have just gravitated to people we have more in common with. For example, my group contains me and two others who are a bit older than most of the other students and commute in. Some groups are mainly international students. That sort of thing.
I would say I am fairly confident in myself- for example I do one optional module with one other friend and if she's not in, I am happy to sit by myself. I don't quite have the level of confidence to talk to strangers (even those who look a bit lonely) in a lot of contexts. I did do it at freshers, and will occassionally do it, but I'm not brave enough to do it all the time. I guess in some ways that must make me come across as cliquey.
I think a lot of people do struggle with percieved cliques at uni. The problem is that making friends really relies on you putting yourself out there in the first few weeks, making a big effort and saying yes to everything, and maybe joining a few societies too. For some people, this is obviously really difficult on top of moving out for the first time and everything else. But as friendship groups get more established, they have to do more of the running, until they feel like everyone is actively excluding them. Most of the time, I would say they're not, but they also aren't in "strike up a conversation with every stranger they meet" mode anymore.
Confidence probably does have a lot to do with it. I made friends easily possibly because I didn't care that much about making friends. I also still have the group of friends I had from my town before going to uni, and meet up with them semi-regularly. So I wasn't desperate for a support network like some people are.
I do think some people believe they are entitled to friendship whilst putting in exactly no effort. I mostly drop these people because I assume they don't want to be my friend if they never reciprocate to invites and stuff, and then they can get upset. However, obviously groups do exist that get their entertainment from gossiping about and excluding others, so it is difficult.
On the flipside of this, I once cut a man out of my life who was very sexually pushy, and was called a bitch and told I was an awful person for this. I don't really care that he's socially awkward, or depressed or whatever. I had broken up with his friend a few months before, so there really wasn't a reason for me to keep seeing him, and I didn't enjoy his company. I don't think anyone has a duty to be friends with anyone else.