[butlike]

Entropy is probably why.

Also as particles spread out, there's less of a chance of interaction, which by extension makes it so there's less of a chance that a system of electrical impulse that fires in a synapse would exist over large distances. This system, of course, would be the catalyst for producing such thoughts as "I wonder if we're alone in the universe."

That's not to say you can't send dense information over larger distances with a shorter wavelength (re: radiation; gamma rays)... it just means the flying saucers with little green men landing on earth are probably out.