Take e.g. Multi World interpretation of QM, it claims there is no collapse of of wave function but it is branching into different worlds, our perception is just limited to a single world. Multi World thus results in gargantuan Universe. It is not excluded that at some point somebody proves the Multi World results in logical contradiction of some kind or that such Universe is impossible on physical grounds, the same could be with other interpretations. This preprint provides condensed overview of different interpretations and some state-of-the-art issues as of June 9th 2021. Returning to the original question: Constancy of the speed of light in vacuum is a postulate ('axiom') of the relativity theory, so don't ask why. Speed of light is also simply related to the vacuum magnetic permeability and electrostatic permittivity in the classical theory of electromagnetism and thus constant, so rather ask why vacuum has those properties with those specific values and no classical theory can give any hint on this. Now comes the quantum theory in its heavyweight edition: Quantum Field Theory in which vacuum is an exquisitely special quantum object with shockingly rich structure. Knowing its properties one can attempt to calculate vacuum permeability and permittivity from the picture and tools provided by the theory. The 2013 paper The quantum vacuum as the origin of the speed of light provides such calculations in apparently good agreement with established values. But it also predicts tiny, tiny fluctuations in the speed of light due to how photon is propagating through quantum vacuum. However, Quantum Field Theory is so intricate that one should consult opinions of a bunch of superexperts what they think about this paper before earmarking the original question answered, or maybe better to say clarified since then immediately questions like why the quantum vacuum is such as it is pop out.