NH3 does react with a group 1 metal (can't think of any g2 off the top of my head). For example:
2NH3 + 2Na --> 2NaNH2 +H2
I think what you meant is why is it that ammonia does not form a coordinated complexes with G1 and G2 metals? In coordinated chemistry, ligands typically form complexes with metals so that the central atom can have 12 or 18 electrons (lots of exceptions). It is usually a transition metal with valence d-electrons. To be honest, I feel like a lot of this is beyond the scope of what you're expect to know from g. chem . But, if you wish to learn more, look up crystal field theory and spectrochemical series.