In BC, this wasn't the case. The raid factions took months to do, and Sha'tari Skyguard, Ogri'la, and Shattered Sun took months. Only the factions tied to the 5-mans could be grinded that way, and normal stopped giving rep for them at revered, which means you were daily locked.
In wrath, most of the leveling factions had tabards you could buy to level through dungeons( level 80 dungeons ONLY, so mostly heroics), except for Kalu'ak and the Sholozar factions, which were tied to about 3 weeks of dailies to hit exalted, and Hodir, which you could farm Shards to gain additional xp for. Raid faction as well.
Cataclysm had the leveling factions with tabards. the Tol Barad faction, and the Firelands faction both took weeks of farming.
Most of Pandaland's factions were tied to dailies, Paragon 5-man bonuses, and eventually the Tiller farm repeatable turnins, and rep tokens. Then you had Emperor Shaohao, who's grind became the standard for WoD, Lorewalkers, which actually could be knocked out in a few hours, Thunder Isle reps, which took weeks, same with raid factions, and Nat Pagle, which is capped at 750 rep per day.
WoD's factions were entirely dependant on how bored you were and how many people you were competing against, since you were slaughtering literally thousands of mobs per faction to hit exalted, but you could do each one in a day or so.
No faction in Legion or BFA could be grinded out in a few days.
The point is, MORE THAN HALF the factions in the game took time to grind out, and still do. The Dungeon tabard factions usually ended up being limited by the daily lockout, and most of the others had daily caps set by the number of quests available.
Edit: I started in BC, so I lived these rep grinds. Also, Vanilla reps took days to hit exalted with? Not in Vanilla they didn't.