Why does halo five multiplayer require Internet to even access it
Because how else would you connect to other players across the world?
Not sure the point of your question there, OP.
There is no local multiplayer (split screen, system link, etc.) on Halo 5, so I guess 343 Industries thought there is no need for local infrastructure (I guess you are referring to playing forge and custom games offline by yourself).
Seeing as you need an internet connection to run xbox gold, which opens you up to the match making of any game, I think the internet is a pretty vital piece of to online match making. Without the internet no one would be able to play match making across all games not just Halo 5. I don’t really know what’s the point of the question.
While we’re here, why do video games require electricity?