Short and Sweet

In general, under most conditions, if you do NOT accept/believe that which you hear, see, feel, sense (either from "out there" or from "in here") as being "Real/Correct", you may be able to "come up with" a more intelligent response mentally, emotionally and physically (and yes, of course, all three do have an "intelligence" that can/should be improved.)

Usually, you have to EXPERIENCE this for yourself, before you can DO IT naturally, most or all the time, because it is not natural. Just look around at others, and at yourself internally. First reactions (thoughts, feelings, sensations) are quite the NORM (read: ordinary, and we've all heard it all before.) What that means is this: humans are made to process impressions ONLY UP TO A POINT, and no further, because it is not required for NORMAL (read: ordinary) life.

But, do know this: all humans are certainly made to be ABLE to experience this - to process all impressions beyond the ordinary level - and to DO IT naturally. Then, the whole world opens up. One can see, feel, experience the Whole World AS Oneself, and isn't that really the most Natural Experience for a Natural Man?

So, what is stopping people from discovering this

sooner than later, or worse, never?

Ahhh, that is the Short and Sweet part of this bit, like the punchline in a joke you don't get at first. Ordinarily you have to see, hear, feel the entire JOKE ("take in" the entire "message") before you GET the joke, at least until you no longer have to wait so long. Being able to DO THAT, consistently, requires activating the higher ends of the brain that thinks in words, thoughts and ideas, in order to "peek through" that which was offered up initially, to SEE THE NEW.

Consider: that which is offered-up-initially, is a WALL, but there are many "points/holes" through which one can SEE THROUGH that wall, which means "getting small", a concept that may require yet another "message" to begin to fully appreciate, but - as suggested - don't accept/believe it as being "Real/Correct", until you can come up with it on your own.

Don't give up, make another effort, and then another. 

Think before speaking/feeling the first thought/emotion that arises.