So the President does all foreign policy himself? He is personally negotiating with the Chinese over tariffs, he doesn't have diplomats negotiating on a day to day basis on his behalf?
This is what you are ignoring. The issue here is that every country has their own way of doing things and their own traditions and etiquette. It is nearly impossible for the President to be versed in how "business is done" in various countries -- what faux pas he needs to avoid and what etiquette he should use to help facilitate them making a deal.
This is where the diplomatic corp comes in. Yes, the President sets foreign policy -- he tells his diplomats what he wants, how he wants things handled, etc. The diplomats then use their knowledge of the country, of how things work, and the etiquette of the country, in making the President's foreign policy work.
Not having these diplomats hurts the efforts of the US in those countries, particularly when Presidents -- not understanding customs and etiquette -- inadvertently offend people in that country.
