They did it because they had the technology to do it.. They used something that was already here: a strong,air breathing creature "apes" and then genetically spliced some genes from themselves that were responsible for intelligence etc. & then you have the first humans, most likely of african origins since gold mines dating back 100k years have been found. Every so often we are genetically altered for increased brain capacity etc. to be somewhat self sufficient. You have to remember we are now altering genes so to a race millions or billions of years ahead of us creating a living creature to a spcific design would be like a human now assembling a chair.