If I recall correctly, In Mage Wars, when a mage summons a creature, they're not dragging a creature across time and space into the arena, but instead creating a local representation. The actual [mwcard=MW1C06]Brogan Bloodstone[/mwcard] is still at work somewhere else, but there's a sort of copy of him in the arena today.
This goes double for the extremely powerful cosmic entities like [mwcard=MW1C01]Adramelech, Lord of Fire[/mwcard]. Mages can summon an aspect of the Prince of Infernia, but his actual power is stratospherically greater than what a mage can summon and control. [mwcard=MWSTX2FFC12]Talos[/mwcard], for example, is an animated incarnation of Akiro. Akiro's actual presence in the arena would be a cataclysm.