I also do wonder whether it’s because of promos offering the ebook at $2-$4.

1/2 It's not promos. It's because everyone in the literary world needs to eat, yet writers of original work are the only ones baking the bread to feed them. My ebooks are under $5. Not perma-sale; all the time. I don't make a living at it—not with how few I sell.😅 But $3.50 per book goes to me.

That will certainly contribute to a low return per unit.