In the logical sense you are totally right. However, from my standpoint (which intertwines with a Christian view of it) God had us created before we were born. Where do you think those souls are kept while we wait to be born or even conceived? They are in heaven awaiting a vessel. So technically the fact that the souls are there, doesn't necessarily mean that all the humans whom body they will occupy are on Earth already. So you are only adding to the souls in God's pool of soul-creation when someone dies. Know what I mean?