Since the OP asked why the builders use Sherwin-Williams paint exclusively, I will answer that question first: volume discount.
Since the OP implied there are better paints out there, I will agree and add that there are many good paints, but the best paint for YOUR walls depends on what qualities are most important to you. Here is a good paint review with pros and cons of paint from different manufacturers and different paints within a manufacturer's line:
Best Interior Paints - Editor's Picks