Huh. I do but I didnt realize it.
It is reasonable to split the 5e Fighter into two separate classes, a heavy infantry Knight and a light infantry Skirmisher. The Knight gets the heavy armor, the Skirmisher gets the mobility. The Skirmisher keeps the name "Fighter".
Knight subclasses include: Cavalier, Paladin, Battlemaster, Samurai, and Eldritch Knight. (Eldritch Knight actually dabbles in light Mage Armor with magical mobility, but is a tank.)
Skirmisher subclasses include: Ranger, Champion, Scout, Athlete, and Psi War.
Rogue is special ops.
Monk is all Dex-fighting.
Warlord is military intelligence and inspiration.
I think we have 2e to blame for Fighter being a "big tent" class to cover all possible variations of "fighting man who is only supernatural by association". There was a push to get rid of "extraneous" classes- no longer were you an Assassin, that was a job title! The Complete Fighter's Handbook pushed Myrmidons, Gladiators, Swashbucklers, Samurai and more on us, with varying degrees of success, including the "magical but not really" Savage, many of which really warped the Fighter class in strange ways (I can't wear armor as a Fighter? WTH?).
3e tried to fix this with 1000 prestige classes in name only, which were more about "here's a way to make this concept work because our multiclass system kind of fails at it and despite what the DMG says about modifying classes, nobody wants to embrace alternate class features unless we publish them", but well, 1, that path could lead to madness if the DM wasn't paying attention, and 2, there are always purists who want a Fighter to be a Fighter not a Fighter 2/Rogue 4/Barbarian 2/Ranger 2 (thanks Human Favored Class!) eyeballing a few levels of Fang of Lolth and maybe Acrobat or Highland Stalker afterwards (you might laugh, but I saw someone actually work towards this insane character, and that's only after I talked them out of being a Dread Pirate!).
So while the game
could totally support Gladiator, Cavalier, Assassin, and Scout as separate classes (as they were at various points during the TSR run), I can see why people might not want dozens of classes running around for fear of them either being too good (Factotum?) or mostly useless drek (Complete Warrior Samurai).
Other people want better mechanics but don't want a certain kind of narrative. Take all the people who balked at playing the Swordsage because "warrior shooting fire out of their sword" was not what they wanted to play, despite being perfectly happy with the third-tier PHB Monk (and let's not get started on the endless "Monk OP!" "Monk sucks!" debate, lol.