Well, yes and no. My wife and I made our own FC. But before then, we declined any invitations to other FC's (it's just us 2. No one else.). The down side to joining a FC are intangible, but still relevant, imo. As a member, your exp/fate/seals will be contributing to that FC's stats. You will obviously, be supporting a FC without prejudice after joining. It also implies a commitment/duty that might no be there in a LS via the blog, message of the day, mission, message board, etc. A lot of people prefer to have their individual freedom. Some people prefer to play alone. Some people would rather not someone chatting in their ear as they're solo'ing stuff while other's would. And that's fine, imo. I know it feels like a snuff for them not to join with no solid reason, but it's their choice. And despite how it seems, it's just what they prefer.

Heck, in 1.0, I didn't join a LS until I got one of my characters to level 50 (solo'ed it all the way). It wasn't until I feel that I have something I can provide or that I have a certain amount of aptitude that I decided on joining. Maybe it's that way for them?