Minor Necron Lore issue

In many of the campaign tombworlds it says they will take lifetimes to reach decent recovery and stuff like that, but scarabs, the repair bots, are von Neumann machines that can eat a hive city and spit out a battlefleet as per every codex ever. It wouldn’t take them a month to convert an untouched non necron planet into whatever they wanted. So why is repair taking so long? I get it’s thematic but not very loreful...

@nemesor-xanxas hush! you act like necrons are at the technological apex of all the other civilizations obviously by their lack of defense against crits and weak firepower that cant possibly be the case.