Why are ONLY 1 billion or less people sustainable for thousands of years here on the surface of earth?

The first thing I want to address here is that the title is ONLY true if people insist on living ONLY on the surface of the earth.

This all changes when people live underground. If people lived underground it immediately reduces everything we use to heat or cool by likely 70% to 90% worldwide because once you have the insulation factor of a foot or more of ground between you and the surface, heating and cooling no longer is a problem. However, fresh air might be so people have enough oxygen underground to breathe.

Also, living underground becomes more practical West of the Rockies simply because the water tables tend to be deeper, especially in the deserts the water table might be 1000 or more feet deep or even non-existent to some degree. So, the closer to the surface to some degree the water tables are the less practical living underground might be unless you are using swimming pool technology or building yourself almost a submarine where you are to prevent water from coming into where you are.

The main reason 1 billion is the "Upper limit" on a practical level of living above ground is about having enough farmland to support everyone on the surface. I suppose you could go high rise apartments but most people (especially in earthquake prone areas on the ring of fire) usually prefer not to do that.

So, if winds are low enough, living underground and leaving the surface ONLY For farming and recreation we likely could support I figure between 13 and 14 billion people on the planet. But, it would be a fairly High Tech life for most.

And we would come to the surface mostly for sunning and recreation and farming.

Imagine the whole world ONLY used for farming and parks and recreation on the surface of the planet.

Above Ground cities would become a thing of the past and eventually they might become farmland or parks once again.

But, the way we are presently living here on earth it is not sustainable. This is why we are in the 6th Great Extinction right now. More species are dying in the last 300 to 400 years than have likely now for thousands to millions of years.

And most of the present species extinctions are due to the fact that humans are now about 8 billion strong here on earth.

So, to be practical about all this we presently have 8 times the amount of people that could be sustainable while living on the surface of the earth.

So, something has to give. And right now what is giving is every species (both plant and animal) not a human might be going extinct here on earth at some point during the next 1000 years or less.