Every national government is different. Some have similar patterns based on history and culture. The phrase "prime minister" is most familiar to me from the British system of government. I think other "parliamentary systems" are similar. The United States simply has a different system from that of the United Kingdom. We broke off from England in 1776. Our founding fathers designed a completely new system of government. They used ideas from the British system and invented ideas of their own. The British have a monarch, the Queen. Why doesn't the United States have a queen? We just don't. The British do not have a president. Why don't they? They just don't. The President of the United States is both our head of state and head of government, but is not the chief legislator. Laws are made by Congress. (Congress is our name for our parliament). The President doesn't have any direct role in making laws. (He has to approve them, which gives him a lot of influence, but it is just influence). The prime minister of England is head of government and chief legislator, but not head of state (the Queen is). In the US, there is a position called "Speaker of the House" which has weak similarities to "prime minister." However, the Speaker is much, much less powerful than a prime minister. Like a prime minister, the Speaker is part of the lawmaking body, Congress; is elected by Congress and leads the House, and thus has a lot of power in controlling how laws are passed. Like a prime minister, the Speaker must have the confidence of the House and _theoretically_ could be voted out of their position at any time. But the Speaker is not a part of the executive branch, can't give orders, can't act alone, and is just a leadership position inside Congress. In one sense, the Speaker is the third most important person in US government. If the President dies in office, the Vice President takes over. If the Vice President dies, the Speaker of the House is next in line.

The US have a president because they do not have a parliament like the UK and instead they have a constitution. Hope this helps :)

Why IS THERE no prime minister in the US? -or better- Why does the US not have a prime minister?