Storm Raven
First Post
The assumption isn't that they don't have any of these items or methods, but that they may not, and that whatever they have they probably don't have enough to shlep out all of it without spending a lot of time and energy that they might not want to spend.
I do find it an odd assumption that a reasonably prepared 1e party will have the ways to find and move all treasure of value out of the dungeon.
No one is saying they will move all of the treasure out of the dungeon, just that the amount that won't be found and moved will usually be a fairly small percentage of the total. Any party that doesn't go into a dungeon without some plan for extracting a huge volume of treasure simply hadn't been paying attention. By the time a party was 10th level (and thus playing the G series), they would have almost certainly planned ahead for removing stuff from the dungeon, and had probably gotten very good at rummaging through anything and everything to find whatever discarded piece of wood happened to be the axe handle of a +3 battleaxe or the trigger for a door into the secret treasure room.