Because the 250% summon damage is additive with the existing damage modifiers from Desmodus and Gather Corpse (900-1000%). This results in diminishing returns, giving a whole lot less damage than it seems.
Every Wizard should be using Arcane Explosion as much as possible as is. The damage it provides is very high, and this set makes it even better.
As far as IMC described, the user needs to deal critical damage, not the summons. This is pretty easy as summoners should already be gearing Critical Rate, and Arcane Explosion makes it trivial. For summoners doing DPS in any end game content (raids, challenge mode, etc) the user dealing magic damage in addition to the summons is normal if you want to do competitive DPS.





