You can’t construct a "why" sentence in French exactly like you do in English.

It’s actually pretty similar if you look at it the right way:

Is nature polluted? = La nature est-elle polluée ?

Why [is nature polluted]? = Pourquoi [la nature est-elle polluée] ?

It works the same way with the other ways to ask a question in French:

Est-ce que la nature est polluée ? → Pourquoi [est-ce que la nature est polluée] ?

La nature est polluée ? → Pourquoi [la nature est polluée] ?

So a "why" question in French is just a normally constructed question with a "pourquoi" in front of it.

I found out that if the subject is NOT a pronoun, you must add the pronoun of the subject as well as the subject itself. In this case, since the la nature is the subject but not is a pronoun, you must add elle, a pronoun for the third-person singular (female).

So for example, if the subject is a pronoun, you can just invert the subject and verb:

Vous parlez italien.

Parlez-vous italien?

But in the following case, you need to add a pronoun:

La petite fille veut un vélo.

La petite fille veut-elle un vélo?

Source: https://www.dummies.com/languages/french/how-to-form-a-question-in-french-using-inversion/