For many events, the demographics lean toward age groups where people have jobs with work schedules that aren't known more than a few weeks in advance. The initially planned friend group (e.g., four people) can have little overlap with who is actually free on the event date and actually attends. Also, if the event has assigned seating, people buying their own tickets typically has the adverse outcome that you can't sit together.
Most flights are available at high frequencies (on the order of days, weeks) compared to concerts (once a year or so). You also don't care as much about sitting together on a plane.
I disagree, if you can't get seats with your friends in a concert, you might just not go because the social aspect is part of the experience, but if you can't get neighboring seats on a plane, you'd (or at least I would) just tolerate it since you would still get to be together at the main event (the destination).
