Virtually no teams are made of 100% incompetent people. If you've got a 20 person team with two worthless people on it, it's barely even noticeable from the outside.
If the person is fresh out of college, of course, this is understandable and a senior person will likely be mentoring them, and reviewing their code. But when you have two such people who gravitate to reviewing each other's code, you have a serious problem that may not be immediately apparent.