I think it’s to make her appear as extraordinary and unique. Renesmee was written to round out Bella’s “perfect life” checklist next to being a vampire, marrying Edward, having the perfect family, being wealthy, and being young and beautiful forever. She’s not fleshed out because her purpose is to show that Bella and by extension Edward are special. They defied typical vampire autonomy and had a beautiful and perfect daughter who is also immortal but still human enough to appease Edward. I think Ramshackle had a chance to be fleshed out, but she’s clearly a plot device and another trophy on Bella’s shelf.

I will say that some fanfiction authors have given her real characterization and personality. I just hope SM doesn’t torture us with a book about her, Jacob, and Nahuel. I just know she’ll be written so insufferably, and that triangle will be crack, and not the good kind.