Is there any reason not to set exorbitant war goals?
Solution 1:
I think it's a dormant feature to provide a more detailled warscore calculation at some point. E.g. not fulfilled wargoals could provide negative warscore for you. So taking too many goals would seriously hinder your potential at winning the war.
However as it is currently is, there really is no reason not to select as many war goals as possible.
With Stellaris next expansion revamping warfare massively, we may see some changes.