In a nutshell: corporate greed. The only part of the game that was live service was the paid cosmetics.
At launch, their entire idea of more ‘content’ was just visual cosmetics. If you look at their communications at the time it will all make sense.
They constantly referred to an internal ‘live service’ team separate from the rest of the game, and that team was effectively the ‘cosmetics team’.
People talk about contractors, but this was the real problem. They thought they could get away with barely adding any real content and selling tons of cosmetics.
I’m sorry to burst your bubble 😅.
Rather than try to explain it myself, here’s a post from someone else that does a decent job: https://www.maketecheasier.com/how-progress-bars-work/
Halo was a legit competitor to cod just past a decade ago.
Now you can’t even compare them because COD is bigger than ever while halo is a shadow of its former self.
Infinite really could have been a partial comeback for halo if they had a steady stream of content after launch, but somehow they added even less than most non-live service games.