It is because some abilities are deemed more powerful under the PCs control.
Think of it the other way, a CR 5 monster is supposed to be a challenge for four characters. Adding that CR 5 to the party isn't just an incremental boost, so the level/CR is near impossible to trade one for one into leadership. That is why the table has such an inflated "cost" for monstrous type creatures in comparison to CR.
Also, if the creature has such a high intelligence, chances are it wouldn't want anything to do with you even if it were thankful for the spell being cast. You are an adventurer and will probably get it killed. It is smart enough to know how to live in relative luxury no danger involved, as it is vastly more intelligent than anyone else in the party if not the entire geographical area.