same reason everyone has to recast inner light,inferno, bound armor
Because you are re-zoning, which is basically completely reloading your character in a new area- while in the game you are just 'walking through a door', in reality your character is being completely reloaded in a new location; the same thing happens when you use a Wayshrine.
Same thing used to happen in EQ2 with Conjurers - then SOE patched it and it never happened again. Just one of those things that slipped through the cracks when they released the game unfortunately.