I think maybe some of this was meant for the comment above me.

That said I think the "I" really is the source of some if not all of these confusions and:

The big difference is that the first question can be easily rephrased to "why are people made out of carbon and not of helium", but the second can't. But that difference isn't enough to make the second tautological or meaningless.

I think the difference is exactly enough to make the second one tautological or meaningless. What you have to do is identify some characteristics of "I" and then ask: Why do entities of this type exist in the 20th century, as opposed to the 30th? If you have identified features that distinguish 20th century people from 30th century people you will have asked something interesting and meaningful.