The aliens are spying on us. If you say "no such things as aliens, noob." Then you are maybe wrong. 50/50. That's probably what they WANT you to think. And they are succeeding. Then, Once we've forgotten about them, or something. They make contact. Once they've made contact, They might send a little probe or something. The probe might say something like: "Hello." Then if our reply is blasting them to kingdom come, Then they attack. If they have advanced technology like that, They ARE smart. And if we don't communicate with them, They might send something bigger. That's my explanation. If aliens are THAT smart, Like, If they have that kind of advanced technology, Then i would suggest not to mess with them.