There are a handful of things that fans consider synonymous with the Halo franchise, one of which is the campaign co-op. Nearly every mainline Halo installment since 2001's Halo: Combat Evolved has had co-op gameplay, which is why fans were bummed to find out that Halo Infinite would not have one at launch. The good news is that 343 Industries has promised that campaign co-op is coming to Halo Infinite eventually. Unfortunately, it appears that this eventuality is going to take a lot longer than the initially proposed May 2022 start date.
343 delayed the campaign co-op and Forge mode of Halo Infinite to focus more on the game's then-impending release. Although fans weren't exactly happy about the lengthy delay, Halo Infinite's positive reviews at launch and subsequent updates helped ease the pain. Not to mention, 343 had promised earlier that the game's campaign co-op would arrive in May 2022, just in time for the game's second season.
Now, according to 343, implementing co-op gameplay into Halo Infinite is a lot harder than they expected, so they'll need more time to make sure that the feature works flawlessly on both the Xbox One and Xbox Series S/X as well as the PC.