With the amount of sophistication game development is at right now. Don't you think it would be easy for them to populate the maps with enemies? I mean I used to make multiplayer maps for Half Life. I could also make single player maps, and pupulate them with enemies. Assign them a area to patrol. And even add a little bit of scripting. Me as a single person could do that. Do you think a huge team of developers like Bungie could not?