Here's why:
* Acoustics: The semi-circular shape allowed for excellent acoustics. Sound could travel naturally and evenly to all audience members, ensuring everyone could hear the actors clearly.
* Visibility: The upward-sloping tiers of seats gave everyone a good view of the stage, regardless of their position.
* Symbolic meaning: The semi-circular shape was often associated with the Greek gods and the universe, symbolizing the harmony and order of the cosmos.
* Practicality: The natural slope of hillsides provided a ready-made structure for the seating, making construction more efficient.
It's important to note that while most Greek theaters were semi-circular, there were also some that had a more rectangular or even square shape. However, the semi-circular shape was the most common and was considered the ideal form for a Greek theater.