Science discovers basic identities. But, the identities it discovers just are the way things are. Why is a thing, the thing it is? It just is. As Bishop Butler put it: ‘Every thing is what it is, and not another thing’.
This sounds mysterious, but it is not.
Why is visible light actually electromagnetic radiation rather than something else entirely? Why is temperature mean molecular kinetic energy, rather than something else? Science does not offer explanations for basic identities. Rather, the discovery is that two descriptions refer to one and the same thing; or that two different measuring instruments measure in fact one and the same thing. There is no basic set of laws from which to derive that visible light is electromagnetic radiation or temperature is mean molecular kinetic energy.
Why is Venus Venus? Why is the Morning Star identical to the Evening Star? It just is.