And how exactly do they know it stays the same weight?
We use up resources so you would think it would get lighter, but then again the population increases so that would add weight.
It's enough to make your head explode.
Google John Michell, Henry Cavendish, and maybe gravitational constant.
Then, read “A Short History of Nearly Everything” by Bill Bryson and all your questions about pretty much everything related to, well, everything will be answered in a moderately easy to read, and very entertaining format.