Answer:
Ok, the rules of the exponent come from a logic construction.
If we have x^n
this means that n is multiplied by itself n times.
If we decompose n into a + b, we have:
x by itself a times, and then x by itself b times, and for how the product works, this is equivalent:
if n = 5, a= 2 and b = 3
x^5 = (x*x*x*x*x) 5 times-
x^5 = x^(2 + 3) = (x^2)*(x^3) = (x*x*)*(x*x*x) = x*x*x*x*x = x^5
And the same for the other rules:
(x^n)^b = x^n*b and such.
Obviusly, this only works when we have a common base.
So the correct answer is that we constructed the exponential rules in a way that only can be used when we have a common base, and this happens because to construct them, we started with common bases.