Venser reached Mirrodin's core with Elspeth and Koth while Karn was trapped in Phyrexian corruption. He gave his own life and spark to free the golem. The gift restored Karn, not the world already being lost around them.
Sheoldred's forces later recovered Venser's corpse and incorporated it into a war construct. A body could be useful material to Phyrexia without containing its former owner's living will. That puppet was not Venser deciding to join the force he had resisted.
Years later, Karn and Melira spent the remaining gift to help restore Ajani and Nissa. Their recovery did not bring Venser back. Karn survived without a spark, leaving his friend's act consequential without making its original cost disappear.