This is a thing with all popular fiction set in the future: in the future from the fiction, what happened to the cultural impact of the fiction? Why has nobody the Doctor encounters ever seen Doctor Who? I recall reading somewhere that Star Trek explained this - once the government realized the fiction was coming true, they classified it all. I have a couple of possible explanations for Halo.

Theory #1 is that Halo was never made. This explains a few things - it's why nobody points it out, for one thing. It also explains why non-AI computer tech seems limited (i.e. 1980s-style computers on warships). Because Halo didn't exist, the Xbox failed and the console market didn't play out the same. With tech companies less successful, progress slowed and that was that.
Without inspiration from games like Halo, military tech took different paths, hence the prevalence of 21C-style firearms and equipment in the 26th century.

My other, more sinister, option is that Halo did exist, but as in Star Trek, was classified when things began to match up. People with knowledge of Halo work for ONI - ONI knows what's going to happen, because it's predicted in the fiction. This explains ONI's secrecy, along with how Cortana found Alpha Halo. It also has some disturbing implications regarding ONI's failure to prevent the whole thing, given their foreknowledge of events.

Also, the Assembly is absolutely the last remnants of Bungie Studios.

What do you guys think?