Because it isn't designed to do so, nor will it ever be, at least not in the way you want it to.

You are asking too much of a single physical theory, and perhaps not realizing that no single theory can ever explain ALL aspects of reality at any arbitrary scale. Big Bang theory will help us answer the BIG questions of the general conditions of matter, space and time. It will, hopefully, tell us why certain physical constants are as they are, and how galaxies were formed. It will not be able to tell us how individual stars and planets formed because that is covered by a second rank of physical theories which only depend on very local physics at a scale millions of times smaller than a galaxy in size.

Big Bang will tell us why certain 'life' issues were settled by nature the way they were such as the initial hydrogen to helium abundance ration, and the age of the universe being comfortably longer than it takes for chemistry to create living systems. But the details of how these chemistries led to life...and particular sentient life...are not covered by the physics of Big Bang theory because the time scales and length scales that are relevant to life are trillions upon trillions of times SMALLER than the scales covered by Big Bang theory.