Interpreting your question to be about introspective analysis, this question seems be unanswerable -- the roots of our knowledge are pre-conscious.
Infants have some kinds of knowledge, and yet, as far as we can tell, are not conscious of that knowledge in the manner that a more mature human can access.
Similarly, there is no "I" there to ask why during the phases of sleep where we are not conscious. Thus trying to chase down the roots of knowledge lead back to these non-conscious forms of knowledge which are unable to be investigated via (dialectic) introspection.
Maybe meditative techniques provide ways of accessing these pre-conscious forms of knowledge but the resulting insights are inexpressible in language, and psychology/neuroscience provides at least some objective descriptions of aspects of these forms of pre-conscious information processing, but that information is not experiential. However, neither of these approaches to understanding the roots of knowledge are philosophical.