Why does the Future reveal that likely only less than 1 billion people remain on earth as of 2100 AD?

Because other outcomes create the complete extinction of the human race and most living things that live on earth otherwise.

So, mankind reducing down to less than 1 billion people on earth is the ONLY way present for any life at all to remain on earth on into the next century. This ecologically speaking is the ONLY way humans survive into the next century at all.

1 billion people on earth is theoretically the most people sustainable living on the surface of earth ongoing for thousands to millions of years.

By God's Grace 

Note: Even though it is also true that 14 billion people could live on earth if EVERYONE lived underground and all cities were torn down and the surface of the earth was ONLY used for farming and recreation and that's all.

However, the likelihood that this could happen this century is very low. So, the 1 billion people living on the surface of the earth is the most likely outcome by 2100 AD.

Even though many might choose to live underground which greatly reduces the amount of heating and air conditioning that would be needed for humans to survive. So, living underground reduces greenhouse gas emissions too.