Why do presidents only serve two terms in office?

Answer 1

Persons are not allowed to be elected to more than two terms as president due to the ratification of the 22nd amendment to the US Constitution.
George Washington set the precedent when he refused to run for a third term in 1796.

Answer 2

The Twenty-second Amendment to the US Constitution sets a limit of two terms for the President of the United States. The amendment was passed on March 21, 1947. Prior to the four terms of Franklin D. Roosevelt, there were only occasional efforts for any President to seek a third term - Ulysses S. Grant sought a third term in 1880, having previously served between 1869 and 1877, while Theodore Roosevelt, who acceded to the Presidency following the assassination of William McKinley in 1901, and served to 1909, sought a third, non-consecutive term in 1912, losing to Woodrow Wilson.

Answer 3

Nowadays there is a constitutional amendment, the 22nd, that limits presidents to at most two complete terms. Before the amendment, there was a custom , broken by Franklin Roosevelt, that two terms was the limit.

Answer 4

Because the US Constitution (22nd amendment) says so. It is the law.