Magnesium or Aluminium hydroxides do not tackle with the root cause of acidity that is they reduce the acidity by reacting with excess acid present in the stomach [neutralization reaction].

On the other hand cimetidine and ranitidine relieve acidity by preventing interaction of histamine with receptors of stomach walls and hence release of excess acid by stomach walls is stopped. So the root cause of acidity is totally eliminated.

Therefore cimetidine and ranitidine relieve the acidity by tackling the root cause whereas magnesium or aluminum hydroxide just relieves by healing only the mere cause of acidity.