It seems you don't have much of an understanding for the character Cortana, so your confused yourself.
And then
You also say that they just revealed everything about the forerunners
Lastly, don't forget this is just the first part of a 3 part series. They can't just throw out the entire storyboard for the next two games in this one, it doesn't work like that. Chances are things will be explained in Halo 5. You could use this reasoning with Halo Combat Evolved, what's the Covenant's motivation for wanting to blow everything up, there is no build up, etc. but thanks to Halo 2 and 3 this was explained, so we can expect Halo 5 and 6 to do the same.