I think there is a confusion that exists in the minds of people about the word “spirit” and the word “psychic.” There is the “spiritual” world and spiritual perception of it, and there is the “psychic” world and psychic perception of it. In spiritualism, the word “spirit” realm is actually the “psychic” realm of many if not most other writing and teachings. So, unless you are speaking from a spiritualists perspective , e.g. mediumship and such, then your question should really read: “Why can’t we see the psychic realm.” The psychic realm is the world or level of this world on which, for instance, auras are perceived, and in which the so-called “dead” find themselves immediately after withdrawal from the physical body. It is also the level on which thoughts and feelings are shared as in ESP or telepathy. This world—which is really just a part of or a dimension of the ordinary world—contains illusory counterparts of things physical, but these are fluid “matrix-world” type things, energy patterns, and not real dense physical things. This world is all around us an interpenetrates the physical world.
Behind that and deeper or “higher” up are another always-present now realms of “spirit” and “soul” and these are not phenomenal in physical or psychic sense, have no close analogs in the physical world, and can only be understood by non-verbal direct experience in consciousness. Spiritual perception is mystical perception and is above or transcendent to physical and psychic perception. This world can not be adequately described in words and is only hinted at in some of the best music, poetry, and art.
Many people have occasional glimpses of the psychic word or things belonging to it such as auras, but most people do not, or if they do only rarely. Some people are born with the ability to see it to some degree and this can be occasionally, frequently, or in rare cases consistently at will.The reason why only a minority can see it with any degree of frequency while most do not, is physiological or more exactly etheric-physiological. Numerous noted psychics of the more rare reliable kind describe what is termed an “etheric web” which in most people acts as barrier between the physical brain consciousness of the psychic world. This web is a natural protection that prevents a premature and chaotic consciousness of the psychic world—a consciousness that most people would be unprepared to handle as they can hardly handle ordinary life awareness without going a little crazy. See: The this second of the book The Grand Illusion: A Synthesis of Science and Spirituality
Some people come into incarnation with an etheric web thiner and so permits (usually only occasional) psychic perceptions. Certain types of meditation also tend to thin the web, and some drugs or traumas can cause breaks in it. The degree of awaking of kundalini has a dramatic effect on thinning or removing the web (see for instance Living with Kundalini: The Autobiography of Gopi Krishna).
The etheric web exists also on a planetary level and it may be the denotation of atomic weapons that greatly accelerated the phenomena of lights and apparent objects in the sky that people identify as UFOS. If this is correct, then most UFOS are actually temporary breaks in the web that precipitate the energy phenomena from the psychic world.
So the bottom line answer as to why people usually don’t see is that they are built that way, it is part of the normal relationship between the physical and metaphysical anatomy—these two being parts of one matter-energy matrix that is evolving. But the chosen path in life can alter this. The natural division between the worlds is decreasing as humanity evolves, and eventually psychic seeing will be as common as are the current ordinary perceptions of the physical world.