Because a Warlord is wider than a single subclass. It's like saying we don't need the Rogue class when we can just have a Thief subclass for the Fighter. Or trying to cram all the Bard college concept into a single Wizard subclass alongside the 8 existing Wizard subclasses. Again,
my own Warlord has 8 subclasses, with a 9th I'm working on:
The Ardent Soul, a Psionic Warlord who fills his allies with his inner fire
The Ballistarius, a Warlord who specializes in leading ranged attack squads
The Borderland Marshal, a Warlord who is used to limited ressources and skirmishing in the wild
The Chosen One, a Warlord with a divine spark in their soul that pushes them toward becoming a figure of legends
The Rabble Rouser, a Warlord used to facing superior forces and using dirty tricks to overcome obstacles
The Silverblade Captain, a Warlord who leads anti-caster squads and inquisitors
The Steel Protector, a Combat Medic Warlord who fights on the frontline and support their allies more than lead them
The White Raven Tactician, the classically trained Warlord and master of formations
And the new one:
The Crimson Sparrow Herald, a Warlord who can lead either a platoon of warrior or a diplomatic mission
Do you really think the Chosen One, and the Arcane Archer feel like the same class? The White Raven Tactician and the Champion? At that point the Fighter might as well be a blank slate with a few proficiencies and extra attack progression and nothing else.
plus, I feel like a Fighter is a more self-centered concept and suite of core abilities, while a Warlord‘s core abilities would be more allies-centered. The disconnect would be too jarring.