Question

Why do modern writers sometimes weave parts of old myths into new works??

Answer 1

Because it makes it more interesting and sometimes they write about myths

Answer 2

The answer is; To give depth to the new work

Providing old myths into new works would help the readers understand why a certain character in the new work choose to pursue a certain action.

The logical reasoning between the myth as a cause that lead to the character's action would force the readers to pay attention to small details of the myth. This will give more depth to the new work.