For those who say don't rely on shouts, rely on a link shell, how did you get into that linkshell? How did you prove to those people that you're worthy of bringing into the fold?

Sure anyone can join a social or casual LS but I'm sure a lot of people have joined a LS from a shout group. Shout groups (or pugs) are the only way to meet like minded people, you bump into the same faces a few times. You ask for a shell and boom you have that linkshell to help you out.

Imagine being a new player now? There's a huge divide between veterans and noobs that those noobs have to break through. How can they get into these linkshells? There aren't enough people in the same boat as them to make their own, and an established progression or endgame LS might be reluctant to take in a total newbie.

So the newbie gets to a natural stopping point where progress grinds to a halt and quits.

TL;DR shout groups are an important social activity that help build bonds between players.