Because you have free will.

You do not see what you do not believe.

I have had many people, at the beginning of the journey, including me, not be able to see. And at the end of the journey they see much more.

One particular thing that you probably know about is auras. Do you see auras around people?
There is a lot more of our world that is even stranger than auras.

And no, you do not get to see auras by saying, "Ok, i now believe in auras." (although for some it might work) You usually have to train your eyes to focus where they would not normally focus, and training your brain to accept inputs from your 3rd eye, and then a learning to focus, and controlling the layers you see.