Why is the great barrier reef considered the largest living structure

Because it is 2,300 kilometer long.

Explanation:

The Great Barrier Reef is located in Australia and is 2,300 kilometers long.

It is considered the largest living structure because is made of corals, which are known as ocean living animals; corals are rare animals that are like plants and are the house of many fish. They are alive and move, protecting the fish that live in them in exchange of cleaning.