While I always appreciate Huxley's writing and attempts of finding unifying principles, I think at the end of the day the explanation is quite simple: the perennial philosophy exists simply because all of these religions originate in the human mind and in human interactions with others and the environment.

Regardless of where/when people come about, our shared humanity is strong enough to lead us to have similar experiences and (somewhat) similar explanations for what we experience.

On an unrelated note, I would like to recommend Huxley's Island as a nice outline of what a realistic utopia might look like.