Think about it:
What is a SUM value? It's a single value which is all the values added together - it's called an Aggregate Function because it aggregates a list of values into a single value.
What is a AVG value? It's a single value which is the average of the values added together - it's also an Aggregate Function because it aggregates a list of values into a single value.
So why would you expect to be able to take an average of a sum?
double d = Average(1 + 2 + 3);
is meaningless in every language, because it is the equivalent of taking the values 1, 2, and 3, adding them together and then dividing the result by one...