Because... it leads to situations like this.
"Your wizards gather 9,728 runes and begin casting, and the spell succeeds. Our mages created 0 acres more land for us to use."
Why allow paradise to be cast if there is no explore pool left to use up? At least in the explore page it shows this information:
"Available Uncharted Acres 0 acres
Alas, there is no more land available to explore in our kingdom!"
Suggestion: Either show the explore pool (maybe call it paradise pool in the mystic page) while in the mystics page so that people know just exactly how many acres it is possible for them to paradise OR do not allow paradise spell to be cast if explore pool is empty. This would prevent the less observant people from wasting runes on a 0 return spell when the pool is empty.