I feel that more thematizing work goes into single-player levels than for multiplayer ones... in single-player the level is often part of a pack, so there are opportunities for the player to get/have gotten thematically suspect weapons in other levels instead, and people usually choose between crates or barrels depending on how civilized the level/tileset theme is, whereas in multiplayer most people probably don't know how to get a generating crate to work well. There's also a strong tradition behind every multiplayer level having a few powerups.