Why does it make sense that six billion people can stretch around the equator?

Answer 1

There is no "why", because it doesn't make sense.

The equatorial circumference of the Earth is about 24,900 miles.

If you try to arrange six billion things along that line, you can only

allow an average of about 1/4 inch per each ... hardly a "stretch".

Answer 2

6 billion people could stretch around the equator more than 200 times. (The distance around the equator is about 40 000 km).