Why We say Alexander the Great not Alexander the greatest.
We can say The most powerful, the most beautiful , most wonderful of something.
‘Great’ is the title he was given historically.
Many monarchs were given such titles, depending on the impression they left people with, especially if there were more than one monarch with that name. The examples below are British, but other nationalities did likewise:
William the Conqueror
Edward the Magnificent
Edgar the Peaceable
Alfred the Great
Edward the Martyr
Ethelred the Unready
Edward the Confessor