I have this head cannon for why the archivists didn’t go and search for the collector! One or perhaps two of the older archivists told the other two, that they vanished, or died while those two were protecting him, as the collector didn’t enjoy violence, the archivists said they passed from the titans power, the other two were a bit younger, like the middle kids, and so they both believed the older archivists. The older archivists said this because they thought the collector was too childish and would always get in there way, so while they knew he wasn’t dead, they didn’t enjoy collectors presence as much as a (caring) sibling would.