Spiritual But Not Religious

Most people who claim to feel spirituality, cannot describe more than the symptoms as they occur in themselves. Beyond that, in terms of what it is, the definition or description and of course, a rational explanation of its origin, one just gets a blank look or vague examples of mundane decisions for which ‘spiritualism’ is determined as the cause or guiding ‘force’.
The question is, why do so many people sense there is more than ‘just’ the physical world? (As if that, in all its incredible scope and diversity were not enough.)