Because science got it wrong at first.

In the 1930s into the 40s, Rudoph Schenkel, a Swiss Animal Behaviorist, studies a pack of captive wolves, and concludes that wolves in a pack fight to gain dominance, with the winner becoming the alpha. Other studies confirmed his findings: unrelated wolves in captive environments do fight for social hierarchy. However, Schenkel's theory was incorrectly applied to wolf packs in the wild for years and years, and was spread far and wide to the public as “fact.”