Besides the equivalence, computable functions are about more than just decision problems. Computable functions are a subset of N -> N. I suppose the confusion is that undecidability (very similar to uncomputability) regards decision problems of the form N -> Bool. This class of functions is also what P vs NP, and general complexity theory tends to focus on.

But the question "is this function computable" is sensible to ask for any function N -> N.