Okay. Stupid question, stupid answer.
But it is vaguely possible that the third dimension is nothing more than an illusion and we're actually living in flatland. The explanation involves some really complex math, which I really am not even going to try to get (I'm not a working scientist).
As with everything weird about the universe, though, it all boils down to the craziness that is quantum mechanics. It also probably doesn't make much practical difference - unlike recent breakthroughs in quantum computing.