Why do you think the constitution states that the president must seek approval from the senate for most political appointments and treaties?

If the president could make treaties and appointments on his own without seeking approval from any other body, he/she could make their best buddy from college the Secretary of State, despite them possibly knowing nothing of the U.S. or foreign governments.  Or, the president could make a treaty with a country that everyone in America dislikes, only because that country promised to sell the president land in exchange for a treaty.  By going through the Senate, treaties and appointments are filtered to ensure that qualified and reasonable appointments and agreements are made.