In Fallout 3, specifically, a lot of the junk had to have weight because there was a specific weapon designed to fire junk out of it.
I'd forgotten about that weapon, or maybe I hadn't encountered it. But I feel the junk problem became even worse in Fallout 4 when it then got used as building materials for the base-building part of the game. It meant you couldn't simply discard it. It's why I ended up having to mod the junk weight out of it. This is an issue that's come about from using something that was never meant to be picked up since there's so much of it laying around. While making it usable, they never made concessions from an engine point of view to keep people from getting overburdened.