A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
It is not possible to change default (and also have it changed inadvertently).
Pivot table has its own inbuilt logic to decide whether to have SUM as default or COUNT as default. It makes its decision on the basis of available data in your data range on which pivot is based upon.
In general, for number data, it chooses SUM as default and for non-numbers, COUNT.