Disclaimer : Not every important thing in life is so simple to be explainable to a 5 year old.
It just is. There is no why.
Do you know why things fall? Nobody knows, they know how things fall (i.e. how gravity works), but nobody knows why gravity works.
You might find many answers on what the number is, how it is used etc etc. But nobody knows why.
Think of it like that one kid in school who tops the academics and sports too.
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