We also while being able to explain that the Universe evolved from the BB, likewise we do not know the why or how. Also whether the view of whether spacetime is real or just purely a mathematical background against which the equations of GR are solved is in my opinion debatable. Do you accept that space is real? Do you accept that time is real? I see all as non physical reality, and as long as one does not excessively attribute the suppositions as mentioned by Professor Lewis thus, "The mistake people make is to treat space-time as a thing and then make suppositions of what happens in relativity based on this". I see no barrier to accepting spacetime as a non physical reality that has been observed to be warped, twisted and rippled [gravity waves] in the presence of mass/energy.