GUNS n AMMO why does our standard military gun use the m-16?

there are way better guns that can support way better answers. Say for instance the FN-FAL, C7, even the G36 imo are better than the M-16. I understand our military likes to diversify not only rounds 5.56 and 7.62 nato but wouldn't it be better to use just one kick gun and one ammunition standard? your thoughts?