This article by Hoyle and Narlikar implies that it is because our future de Sitter universe is a special case of the steady-state mathematical metrical field. The dark energy density we measure in our past causing our universe to accelerate its expansion is the reciprocal of the area/entropy of our future horizon. Therefore, it must be a perfect future absorber.
"This article reviews the developments in the electrodynamics of direct interparticle action, emphasizing the achievements in quantum as well as classical electrodynamics. It is shown that the application of the Wheeler-Feynman absorber theory of radiation places stringent requirements on the asymptotic future and past light cones of the universe. All Friedmann cosmologies fail to meet these requirements, but the steady-state and the quasi-steady-state models have the right kind of structure to make the theory work. Further, it is shown that the working theory is free from the problems of divergence that trouble the classical and quantum field theory. In particular, no renormalization is needed: The bare mass and bare charge of an electron are finite. A few ideas relating to the response of the universe to a local microscopic experiment are presented as well as on possible clues to the outstanding issues of foundations of quantum theory."