Co-op split-screen is synonymous with Halo as much as the co-op campaign. Unfortunately, 343 Industries confirmed that Halo Infinite will not be getting one of the series' most iconic features. Now, we just found out why.
According to 343 Industries Forge lead designer and head of live service Sean Baron, split-screen co-op was close to completion before 343 decided to cancel it.
As Baron explains: Making sure that the quality of this release is high and good and that we land that. We of course were working on it for many, many months, years, on campaign couch co-op, and there's a lot of challenges there from a technical perspective and what people were able to glitch into is probably about 80% [of Halo Infinite campaign couch co-op]. But the amount of work for us to get from 80% quality to 100% quality is significant, and the amount of effort that would take right now is not something that we can commit to. I'm not even sure how long it would take, to be honest. TLDR; the reason why Halo Infinite doesn't have split-screen couch co-op is 343 can't do it and still stay on schedule.