Presumably because they are continually picking up new players from younger age groups while retaining many older players, but I don’t really have any interest in those games so I don’t know much about them. I’m not really seeing how this directly relates to my comment, either.
Empirically, Final Fantasy 14. Also my only 1000 hour game since games services started logging playtimes in a more durable way. Only other games I can think that might have touched that time are Diablo II and UT99, but both of those playtimes are lost to the sands of time.