A vacuum is the absence of matter. If the physics community decided to define a medium as something made of matter or made of particles of matter, I can't stop them. Nobody is obligated to find a cause for a medium that they can't detect. Personally, I find a medium useful because it gives me a way to interface with space and space-time. It gives me a way to attempt to manipulate space-time using repeating lineary frequency chirps of EM energy. If the idea works, then there will be another way to curve space-time (create gravity fields) besides the stress-energy tensor.