I could buy an argument that is something like the Prime Directive – an alien species might, out of respect, not wish to interfere with our natural development. Perhaps they are waiting until we have “warp drive” or some other threshold technology before we are invited to join the Galactic Federation. In this case, they will not have made contact, be sipping tea with Donald Trump, and trusting that some retired general will not reveal all their secrets. We would simply be completely unaware of their existence. If, on the other hand, they feel it is time to reveal themselves to world leaders, then they probably also feel that we have reasonably stable governments that are in control and can be trusted with this knowledge. This is probably the same threshold as just letting the world know they exist. Why would they ask world leaders (and all their necessary support staff) to lie to their own people?
I have heard dozens of speculations as to why they might do this, none convincing, and I’m confident some will appear in the comments below. They want us to know that nuclear war is bad. No, really? Thanks for the head’s up.