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The 'why' is because 'it's what balances'

i.e. it's the only combination that works. A proton is a bunch of other particles that, when combined together, balance out an electron. The 'why' is 'because that's a stable configuration' in the same way that water at 25c is liquid not gas because the 'rules' of the local environment dictate that.

I mean, why do those particles exist at all? That's really what you're asking. Why do electrons exist, why do protons 'form' from subatomic particles to balance them out? Existential kinda question.