War Mongerer was trying to erase the events that had happened, including the one where Loki took their Core Metacs. But Loki didn't want to lose the Core Metacs, and he definitely didn't want the progress he had made in his conversion efficiency to reverse, so he refused to let the Core Metacs go.
His unwillingness to lose his conversion-efficiency progress is why he didn't put the Core Metacs in the spark of power and call it a day. If he put the Core Metacs in the spark of power, as he had always done before, the Core Metacs would be cut off from the world, and the timeline would be free to reset as War Mongerer wanted.
In that case, he would get to keep the Metacs while War Mongerer got to reverse time. But he didn't want War Mongerer to reverse time, so he held on stubbornly to the Core Metacs, even though it cost him a lot of lifespan to keep them from disappearing.
