"The Kingdom of heaven is within you", it points to the fact of discovering the true SELF within....
When one has the cosmic consciousness, one can feel the cosmic Self as one's own self, one can feel one with other beings in the cosmos, one can feel all the forces of Nature as moving in oneself, all selves one's own self.
There is no why except that it is so, since all is the One.
All is in the SELF; when identified with the universal SELF all is in you. Also the microcosm reproduces the macrocosm - so all is present in each, though all is not expressed (and cannot be) in the surface consciousness.
Everything acts in the SELF. The whole play of Nature takes place in the SELF, in the Divine. The SELF contains the universe.
The SELF lies essentially universal; the individualised SELF is only the universal experienced from or in an individual centre. If what you have realised is not felt to be in all, then it is not the Atman; it is the central being not yet revealing its universal aspect as Atman.
The SELF is felt either as universal, one in all, or as universalised individual the same in essence as others, extended everywhere from each being but centred here. Of course centre is a way of speaking, because no physical centre is usually felt - only all the action take place around the individual.