why can’t they just play and be happy with what they have
Because we're humans.
If there's a better private server with better balance and more fun PvP etc, people will move there.
The only way to have people be loyal to a server for such long time is to give them satisfactual entertainment; once that server lost that factor people will start leaving or move on.
Honestly in recent times I'm finding it stupid that people tends to make their own alli, fail to pk because they get ganked and quit. A dying faction should merge occasionally to keep it alive. What's the point of faction pride if it just get buried in time? Nothing.
Basically, people are stupid because they think they can make a barb and suggest that they can solo the entire game by themselves. This is how I find many people quitting tbh, because they can't be smart enough to stable their factions so that it lasts longer than one hour of pking. TW is the big joy of this game; but I am yet to see two competent factions lasting an elusive war. It's just "OMAGAWSH WE NID MOAR BMS MOR BARBS MOR ARCHERS FOR TW WIN WIN WIN". What kind of repetitive TW is that?