Translation:Why do we assume that he knows everything?
Not sure at all about what I am saying, but maybe annehmen is more about assuming, in a way that you accept it, but it not really your initial consideration or opinion. To assume can even be used sometimes in hypothetical cases, when you suppose something . To consider would be more about defending your opinion. At least that's the difference I see between those two words in English, so I guess in German it's the same.