Because you can convert the USA to fascism, or convince the Soviet Union of the value of despicable neutrality - using a resource called political power, in strictly limited supply. The AI doesn't use the functionality - I presume because it would be frustrating and irritating. (A common feature of espionage systems). The AI also has virtually no ability to defend itself - presumably so the player is actually able to accomplish such significant changes if they want to.
It is actually pretty ironic; it's called the Clausewitz engine,yeah? "War is the continuation of politics by other means." Hearts of Iron embodies that - it is at heart a political, ideological struggle. So does Stellaris. It's a game in the same engine, by the same studio, in the same genre. An ability to meddle with the politics of other empires directly risks trivializing the emergent rivalries that Stellaris currently features. A optimizing player will use it to cut off their strongest opposition at the knees, particularly if they are presumably vulnerable slavers/conquerors. And the AI using these measures against the player will basically be raining on their parade - inhibiting their ability to actually play their empire. Which is a big pile of shit. Even a system that's shackled to limit its impact but still be useful has significant dangers - several of the Total War games featured an 'Agent Death Spiral' when enough nations were at war with you.