Not all monsters in EQ fled - undead, for example, fought to the death. In EQ 'train' behavior was common when a fleeing monster or a pathing bug caused everything in the dungeon to chase after a player. That player would then run for a zone line to escape behind the safety of a loading screen.

In general most developers have noted that players hated this type of gameplay because it meant that one idiot could ruin an evening for 40+ other players in the dungeon because the 'train' would then clobber all the other players on the way back to their individual spawn points.

WoW also generally doesn't have zone lines that you can escape out of. The fleeing mechanic isn't something that's all that challenging to advanced players who know how to stun and slow. It actually makes encounters easier because the monsters stop doing damage at 20% health or whatever. Contemporary games have other ways of adding challenge.