Why is this movie called "The Boy and the heron" in west?

Officials have yet to give a reason for the title change. However, there are several examples of titles from Ghibli's past works that have not been directly translated into Japanese.

Mimi wo Sumaseba (If you listen carefully) -> Whisper of the Heart.
Sen to Chihiro no Kamikakushi (Sen and Chihiro's Spiriting Away) -> Spiritted Away, etc.

You are right; the heron has little to do after entering the other world. However, looking at the story as a whole, you will see that the heron plays an important role.

To put it simply, the heron was an absolute stranger to Mahito, to the point that he initially wanted to kill the heron, but through their heart-to-heart journey, they saw the possibility of becoming friends. And the heron also leaves him with this final word. "See ya, my friend."

It is significant that even the eerie other, the heron, can become a friend through the world of the story (other world, fictional world), which, of course, has a literal meaning. In addition to this, through the world of the story, if a work or character can become a friend of the viewer, even if only for a moment, if it can be close to people, then it has become an anime.　

The story begins and ends with the heron, a significant character that reflects the growth of Mahito.

I am simply offended that there is no bestiality in the fillm even though the title implies otherwise.