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

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.

yes based on alexanders actions he deserved the title of "the great". his empire covered about 3,000 miles and he was able to establish the empire over a 13 year period of time. he was able to conquer many different places. he also defeated the persians. he conquered egypt and developed a city and named it after himself.