It is because operators have a property called "precedence." Precedence, sometimes called "binding strength" or "binding level," determines the order that the operations are done. In mathematics, the exponentiation operation is done before any addition (and, by extension, subtraction) operations are done. Now, negation is actually considered to be subtracting the number from zero. So, -3**4 is actually (0 - (3**4)) when you put in the parentheses sets to disambiguate.
Now, why the operators are evaluated in that order is a more complicated, due the need to understand their iterative nature, and need to understand identity elements and inversion elements.