To state the obvious: Of course you can only make friends when you talk to people. However, most people who want to make friends probably only want to engage in activities that have a moderate to high likelihood of producing friendships.
Now, how many of the people the wallflowers aren't talking to do actually seek any new friends? For one thing, many people want to enjoy a given event with their friends (and only their friends) whom they've brought to the event. That's often the case at concerts or clubs, in my experience. For another thing, people today are highly mobile and - thanks to their smartphone - in constant contact with their "old" friends so that the set of people they really care about doesn't change much anymore after school. After classes at my uni, for example, many fellow students would drive up to 100km home to live with their partners or to hang out with pre-uni friends. Among the rest, "friendships" existed mostly for learning and partying and thus didn't evolve to something deeper.
Some people may prefer to have a few real, very close friends than 100 "friends" that they never talk to. Some people also like to go to events to just be out and about and not at home. I don't feel weird eating dinner or going to a movie alone. I'm there for me and am not out to "make new friends".
It is a little like co-working space. Yeah, I could work at home but there is a lot more energy being at a location full of people being busy even if they aren't my "friends".
Now, how many of the people the wallflowers aren't talking to do actually seek any new friends? For one thing, many people want to enjoy a given event with their friends (and only their friends) whom they've brought to the event. That's often the case at concerts or clubs, in my experience. For another thing, people today are highly mobile and - thanks to their smartphone - in constant contact with their "old" friends so that the set of people they really care about doesn't change much anymore after school. After classes at my uni, for example, many fellow students would drive up to 100km home to live with their partners or to hang out with pre-uni friends. Among the rest, "friendships" existed mostly for learning and partying and thus didn't evolve to something deeper.