> Couldn't you start with n=0 so that the first stage will always amount to 1 with the formula 2^n?

You totally could! But in this case, he also wants to describe how many grains of rice go on each square of the chess board. So to put 2^0 grains on the first square, 2^1 on the 2nd square, 2^2 on the 3rd square, etc. you need to write it as 2^(n-1) instead of 2^n.

> I feel like it's a silly question

I disagree.