Telling people to be more active or "just go play then" isn't going to work. Majority of people like to have incentive to play, as-well as a challenge and the fun of others to play with.
With the huge gap like Tricky recently mentioned, even i couldn't be bothered since mainly all active players are in set top 10 guilds, well top 5 pretty much now. Some people like wipes in games as it gives them the fresh start, feel like a new page has turned. I for one, wouldn't play as much before a wipe, but if one was to happen again i would come back and play more unless of-course it failed miserably.
On to the point of guilds.... most guild events, guilds lead to players not playing with other non-guild members or just people without guilds which in my perspective isn't good. A balance of guild points should be based on kills a player gets x the mob level/sector level / time taken to complete or something. So every player has to earn their guild-points anyone leeching with no kills will only gain exp. Not that this would help with people not playing together, i'm not sure what to suggest on bringing more players together again but in the couple years i've played i have just seen more and more locked, closed, password protected rooms and people just playing solo and or just in a small guild group.