Some of us are old enough to remember when a German F�hrer ("leader") did want to come and rule Britain! Presidents are elected to lead their own country, and will try to do what is best for that country, not the whole world. Quite often, of course, what's best for their country is to co-operate with the rest of the world and then they'll do that, but they won't beggar their country for someone else's benefit.
There are, unfortunately, other presidents of other countries who do very well indeed for themselves, their families and their close companions, while allowing the country as a whole and its people to sink into ruin and starvation. Think Saddam Hussein, Ceaucescu of Romania and the man in Zimbabwe whose name escapes me.