Why should it be assumed that that there is no reality that goes beyond our direct experience, one that simply cannot be sensed and observed?
It doesn't, but
neither does it follow that just because there
may be aspects of reality that can never be observed by our senses, there
are such aspects; and neither does it follow that just because there may be such aspects we can just
make up what those aspects
are and call them "God" or "the supernatural" or what-have-you.
Kantian doubt really offers nothing for the believer to stand on. The Kantian undetectables, by definition,
cannot ever affect us (or else our senses could detect them) therefore they're non-existent for all practical purpose. Certainly for all reasonable purposes.