Orcus with Last Word can insta-kill infinitely x infinitely x infinitely above baseline people with many layers of Mid-Godly and Resistance to Mid-Godly Negation. He also seems to resist most of Thanos' abilites sub-raw damage.
In my view its solely because the LoP's avatar seems beyond the reach of any divine entity. Even if working together. Like Vecna had a nigh-omniscient snake that embodies magic give him dark hidden knowledge of reality warping languages and he was amped using a legendary spell to be a Greater God of ever increasing power and.... he's barely strong enough to hold off the avatar.