I agree but there is a very clear base game play similarity between CE, Halo 2, Halo 3 and Halo 2 Anniversary, and then Reach, Halo 4 and Halo 5.
Aside from different weapons, vehicles, movement speed, aim assist, the only real major changes between CE, 2, 3 are health pack/shielding change after CE, fall damage gone after CE, boardable vehicles and dual wielding, and then equipment in Halo 3.
Then add Reach, you get bloom, plus you now have armor abilities like Sprint, Jetpack, Armor Lock. Then Halo 4 and you now have sprint made universal, perks/specializations that give you things like increased ammo, stability, vehicle ejection. You also have instant respawn, killcams, personal ordnance, global ordnance. Finally with Halo 5, you now have the removal of classic zoom and instead get aim down/iron sights, you have automatics play differently on whether they are scoped, you have universal and unlimited sprint now, thruster, ground pound, charge, hover, slide, killcams return.
Halo 2 Anniversary is basically Halo 4 but without the things people hated like flinch, sprint, Loadouts, perks, ordnance etc.