We did not want to support to add new line because it would require to manage all the line behavior in the amendment (e.g. price computation, tax setup, promotions etc.).
Also changing the quantity of an existing line is OK as at least you have supply for the initial quantity but with a new line you do not know if you have enough quantities at the shipping date.
So it is simpler and provides a better user experience to create a new sale when customer request new products.