Why does the inverse square law apply to gravitational radiation amplitude?

[wolram]

gravitational radiation is now widely accepted, and the mathematical
outcome of an events radiation is calculable, but why is the inverse
square law applicable to the amplitude of this radiation?

 

[Kurdt]

Anything that is spherically symmetric will obey the inverse square law as when it is moving from the centre of origin it is spreading its intensity over the surface of a larger sphere and as we know the surface of the sphere is related to its radius squared.

 

[wolram]

i must be careful here as i don't have the facts in front of me.
i read some time ago that a 5TH power may have a short range
effect on the ISl for gravity, and if this effect is not found then
certain, "extra dimensions" theories can be ruled out, i think the
EOT-WASH group have done tests in the mm range, but I am not
sure if these tests are conclusive.