Why are there so many cabinet departments under the president of the US?

The US president is responsible for running the federal government. The federal government now has its finger into every conceivable aspect of the lives of the 3.13 million people living in the country as well as many concerns all over the world. It take vast numbers of people just to fill the leadership positions in all the agencies and every leader wants as many people under him as possible.

Occasionally somebody tries to eliminate some agencies, but every agency has workers who want to keep their jobs and almost every agency provides some service that is enjoyed by those who get it. People concerned with other agencies empathize and add their support to the threatened departments. These people all combine to make such a protest that nothing happens. Meanwhile politicians think of new services that might attract votes, so the government grows bigger still.