Swamp is the traditional name for "shroom" and easier to say.

Spongy, muddy soil got its name from the little critters that were in it, who were spongy too. However muddy soil under shallow water is properly a fen.

Swamps make for nutritious eating and are a traditional way of meeting the elves and smurfs. If one believes the old stories, they can make the right kind of folks rich.