I'm just guessing, but I think it's probably an anti-cheat device. If you disconnected straight away people could use that to avoid taking hits/debuffs/deaths in certain places and then log back in ~30 seconds later without any harm whatsoever.

One misuse I can think off would be those times when mistakes are made and the boss is going to do some mega hit that wipes everyone except the tank, but if people can just all disconnect before and reconnect after... Harder to coordinate with pick up groups, but preformed groups could use that to their advantage to skip mechanics.

So instead the game keeps us logged in for 5+ minutes to make it completely impractical to force a disconnect. Of course it is annoying when we disconnect for whatever reason, but the game has no way of telling the difference between us doing it and an actual network error.