To exist. You can ask that question about any living being. Why are they surviving? Well, it's more about the genitic code, the genetic materials. Anything from a single cell organisim to humans, all main purpose is to survive. This applies to the living of life itself; as a philosophic stand point, life has no purpose whatsoever. It is simply a consequence of the laws of physics in this universe.
The question really should be, "Why do humans have a need to seek meaning?" To this I would say that I really dont know, but I can offer speculation that the realization that we are going to die makes it necessary for humans to wonder what the purpose in living is if death is going to happen regardless of what they do. Thus, they search for meaning in life. This is simply a consequence.