Here are my beliefs on why some people deny that these things happen: (Remember, they are just my conclusions, and I'm not saying anything negative about anyone here - it's just an interesting subject to me.)
There *are* people who don't (or can't) experience this, simply because they are not "wired" to do so. It's kind of like being colorblind - some are, some aren't, except in this case, more people are *not wired* rather than *wired*.
Animal people usually fall into the range of the more perceptive or the *wired*. At least in my years of observation. Talk to any very successful obedience competitor - they develop such a close relationship with their dogs that they can feel the dog's presence after it passes.
I used to regularly know when Neechee was crated at home, and needed to go out. When in the ring on the long sit or long down exercises (the place where Neechee NQ'd MANY times due to noises or echoes-especially applause-in the venue) I would visualize him glued to the floor or send comforting thoughts at him. Any time I did this, he was perfectly content. If I forgot, he NQ'd. Many of my fellow competitors used this technique to keep themselved psyched up in the ring and quiet their nerves, but who's to know if it was some kind of telepathy or just the heightened sense of the dogs to something we can't see, hear, or smell?
I don't think it will ever be proven or disproven, so I base my beliefs on my experience. Is the cold nose on your leg or the sound of toenails on vinyl flooring really there? Or are you so accustomed to hearing it that your ears or mind plays tricks on you? When new dog acts as if he sees or hears or smells something from old dog, is it really old dog communicating, or is it something we just can't identify? There are so many explanations, believers and non-believers. It comforts me to think that it's my old friend, heart-of-my-heart, letting me know he's OK, and still loves me.