WHY haven't humans tamed zebras?

 In the 19th century, colonizers considered domesticating zebras in Africa as a solution to avoid importing horses to the continent. Zebras are also highly resistant to diseases spread by the tsetse fly. However, unlike horses which are relatively calm, zebras are constantly in a state of stress and easily flee or even attack predators in the savannah. If cornered, a zebra can even defeat a lion in combat.