Image is one of them.
The plane represents the United States.
No other head of state plane is a striking.
Konrad Adenauer-Germany A340
Air India One-India 747-400
France A330
Japan 747-400
Russia Ilyushin Il-96-300PU
Argentina 757-200
Azerbaijan 767-300
Bangladesh 777-300
Bahrain 747-400
Egypt A340-200
Iran A340-300
Iraq 767-200Jordan A340-600
Kuwait A340-500
Libya A340-200
Mexico 787-8
Point being, In a world where image is important, Having a plane like a 747 is important.