Since we can say "Why can we grow taller?", "Why cannot we grow taller?" is a logical and properly written negative. We don't say "Why we can grow taller?" so the construct should not be "Why we cannot grow taller?" The reason is that auxiliaries should come before the subject to make an interrogative. Why should we capitalize the first person pronoun 'I' even when it does not appear at the beginning of a sentence?

Understanding the Context

Why is it not the case for other pronouns?