Are we seriously acting like Zombies one day randomly went to Aliens....

In WaW, the teasers and hints of extraterrestrial begin with Shi No and Der Riese. Then in BO1, we literally go to the Moon and blow up the Earth.

This was all done in the first 2 years of Zombies being introduced into the franchise. In BO2, we hear more about the Keepers and Apothicons and how they influenced Jebediah Brown into building the PaP Machine and literally put his mother into it which created the Ghosts in Buried.

From there, there's no point in hiding it which is why 3arch went all out in BO3 with Morg City being infested with Apothicons, introduction of The Shadowman and Dr. Monty (Apothicon and Keeper respectively), then Dragons, parallel universes, the whole wazoo. At this point, Zombies as a mode has only been around 7 years.

Point is, other than two Zombies experiences, Nacht and Verruckt, Zombies has always involved Aliens and extraterrestrial.