My understanding is that it’s not so much Xbox as it is the game designers. There apparently are Xbox one games that you can LAN with; the system is capable of it, Halo just doesn’t happen to be one of the titles that can.
I've been complaining about this since the launch of the Xbox One. I bought 16 Xbox One's with the impression that I'd be able to continue system link/LAN events but was furious to discover they don't. Microsoft got their way and shipped out a console that is required to be online all the timeYeah that sucks man. If i remember correct, didn't they misrepresent (lie) that they would be LAN compatible? Could be wrong, but it seems like that was the case.
MS as a company didn't support LAN on the Xbox One in the beginning.
Now I don't know the full details, or how it all came about. Maybe MS didn't provide support for adding LAN in to XBox One games, or maybe MS pushed for Devs to remove LAN from their XBO titles, or maybe it was a left over from their stupid always online set up they initially pushed. I'm trying to think back and remember if MS or 343i themselves had actually said that the system doesn't support LAN. Whatever it was, many actually believed that the system couldn't support LAN.
In fact if you look at earlier games, like Borderlands, Dead Island, Halo MCC, they didn't support it, despite having LAN on the 360 (They removed LAN). Then BC games came to the system which supported LAN, and even COD games came out with the feature. Realising that the system can supprt LAN, fans hit back at MS, and it was that bad that when Gears 4 was announced they made it a point to highlight that Gears 4 would feature Split Screen and LAN, two features Halo 5 received heavy backlash for omitting.
Maybe if enough people ask, 343i could add it in with the 2018 update.