Please see Lei's excellent answer in this thread
Credit Lei Chen
Didn't find any official reason, but I believe that sum is one of the most frequently used aggregations.
See more info here, Aggregations
If you hope to change the default behavior for "percentage of cat owners", please also refer to How to Set the Default Aggregation for a Measure .
Credit where credit due! was and still is a great answer... hope all is well
I guess you might have already got the answer.
However would like to add some note on top of that, tableau aggregates the measures because measures are dependent variable. Whenever you drag any measure in your canvas it always gives you a single value always. Whereas dimensions are independent variable which creates header for you.