The driving factor is a pragmatic desire to improve mental capacity. Alien beings may have already reached a point in their evolution where, having exhausted the potential of their biological brains, they have taken the next logical step and opted for robotic brains equipped with artificial intelligence.
This brain swap may not be as far off for humans as one might think. In only a few decades, the computer revolution here on Earth has produced supercomputers capable of performing more than a quadrillion calculations per second. (According to research by Hans Moravec, an artificial-intelligence expert at Carnegie Mellon University, that rate trumps the human brain’s estimated top speed of 100 trillion calculations per second.) Some scientists speculate that in a few decades, an event called the technological singularity will occur, and machines armed with computer brains will become sentient and surpass human intelligence. Civilizations equipped with technology light-years ahead of our own could have already experienced the singularity thousands, or even millions, of years ago.