Because that value has been established by convention. Any number raised to the power of zero equals one. See the related question.
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By definition, any number raised to the zero power is "one".
The inconsistency is the following:Any number raised to the power zero is equal to one.Zero raised to any power is equal to zero.
Any integer raised to the power of zero is 1.
Any non-zero number, raised to the zero power is equal to 1.
Anything (except zero) raised to the power zero is equal to one.