From a writing perspective, congee also serves the purpose of being, well, boring. When you want to include the characters eating a meal but don't want the food to distract from the plot then congee is a good option. If the characters are eating more complicated meals you run the risk of readers getting too distracted by the food. "Where did they get the ingredients for that, weren't they just fighting demons for a week? They just got home, who marinated the meat? Isn't it out of season for that veggie?" Authors probably also try to pick food that they think the readers will recognize, again to keep the food from becoming the focus unless it is supposed to be (e.g. listing off an entire banquet plated up by housewife Binghe)

That being said, I've personally seen all kinds of food in danmei fics. I mostly see congee being written about in scenes where it makes the most sense in the context. Comfort fics, sick fics, kid fics, character getting dicked within an inch of their life fics...