It is hard to argue with American Pharoah's status as one of, if not the great, race horses in the history of the sport. His win at the Haskell Invitational is his eighth straight and the victory did not even appear close. The horse keeps improving from race to race and in doing so continues to wow onlookers.
Many horses have won various titles throughout their career. Just because American Pharoah won a few big races, it does not grant him the title of the best race horse in the history of horse racing. There are many different things to consider when calling someone the best of all time, and American Pharoah does not deserve that title.
I believe it is not possible to call American Pharoah the 'greatest horse in the history of horse racing' because every race is different. To determine the greatest horse would require all the winners to date participating in one race with identical riders. All variables would have to be kept the same to achieve a fair result.