I can think of 0 reasons why the “police should investigate that”
Why?
Why?
That's just the tip of the iceberg here. It needs to be understood why they took this course of action.
In this case it's a bit more suspicious, but it's not an unlikely explanation by itself.
They did. They found messages of the girl self aborting well into viability and then planning to hide the body.
https://www.vox.com/identities/2019/11/8/20954980/stillbirth...
In any case, such nuance is likely beyond the grasp of a 17 year old. I think you are well aware that this doesn't always yield simple seeming edge cases and that you are capable of looking into other examples but chose not to. Here's some more.