Age of Empires 2 came out in 1999, developed by Ensemble Studios. Age of Empires Online was released in 2011, by Robot Entertainment and Gas Powered Games. A good portion of its code came from Age of Empires 2[1], including its terrible path finding[2]. Modern RTS games actually have pretty good path finding, and even Starcraft (1998) had better path finding than the legacy code used in AoE Online (2011). Microsoft published both games, and presumably they dictated the reuse of the old code. Probably to save money.

[1] http://www.quartertothree.com/game-talk/showthread.php?60855... [2] http://www.quartertothree.com/game-talk/showthread.php?60855...