Final answer:
Quintilian calls it the Institute of Oratory because he focuses on the practical aspects of public speaking and the art of persuasion, which was highly valued in ancient Rome, rather than the theoretical and philosophical aspects.
Step-by-step explanation:
Quintilian refers to it as the Institute of Oratory instead of the Institute of Philosophy because he focuses on the art of spoken persuasion and how to effectively communicate as an orator.
In ancient Rome, oratory was highly valued among the aristocratic class as a skill in government, whereas philosophy was less valued. Quintilian's teachings in his Institutes of Oratory focus on the practical aspects of public speaking, such as body language and gestures, rather than the theoretical and philosophical aspects.