Depends a bit on the game.
In Infinite AR starts, people still ignore the Pulse Carbine, Plasma Pistol, Mangler, Needler, Sentinel Beam, and Commando most of the time. There are exceptions, but 99% of what I feel like I see used is ARs, Sidekicks, and BRs, with the occasional Bulldog, Stalker/Shock Rifle, and Power Weapon thrown in.
I see exactly the same thing in BRs, but noone uses the AR or Sidekick in it.
So purely by nature of people actually using ARs and Sidekicks in ARs, it has exactly those two weapons’ more variety, and the one or two guys on the map with a BR in their hands are just better off than everybody else.
In past Halo games, where the AR and Magnum, or SMG in Halo 2’s case, were actually bad, I pretty much saw the same thing, except old Halo maps usually had more than 1 or 2 BR or equivalent spawns to work with. It was a bunch of people running around with their ARs (or SMGs) unless they had a power weapon or got their hands on a BR, Carbine, DMR, or Needle Rifle.
And then of course in Halo CE and Halo 5, everyone just used the Magnum and power weapons. Heck, in H5 the BR, DMR, and Carbine we’re all basically redundant because the Light Rifle was the only precision weapon that actually outperformed the Magnum, with a small argument being made for the DMR in long ranged engagements, but that was very map dependent and mostly only applied to Warzone.