Because of political needs and their own expansion chant.
Ancient emperors didn't just use a name or a title, they had to establish themselves as a unique being. He was appointed by heaven and lived forever.
Want to do this, you must be different from others in the name above, the princess or the king can also call them a thousand years, but the emperor must become a long life, indicating that they are ten thousand people, is the son of the heavens even if the death is only back to the embrace of the heavens, still exists.
This acted as a deterrent in those obscurantist times, and naturally the title was carried over later.
The other is their own self-confidence is inflated, want to tell others their ability to great others can not be compared, and can enjoy a long time in this throne.
Sort of a psychological implication, tell yourself that this throne can sit for a long time, but the facts prove that this is not useful, looking at the history of China, every emperor is much shorter than their own ministers, in addition to a few other examples.
Long live or not, short life or not, the emperor this title eventually disappeared in the river of history.