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Many, many companies offer 3 liter bladders. Only the dromedary units go above that and the reviews say they're hard to clean and in general are negative about using them as hydration bladders rather than just water carriers.
What's about 3L that they figure nobody wants more?
It's all about weight.
Water has a density of 1kg/L. Thus 3L is 3kg or about 6.6 pounds. That's equivalent to a brick on each shoulder when placed inside your pack. That's really not that much, but as you add more and more water it adds more and more fatigue on you during your hike.
For myself, 2L has always been plenty for a day of hiking or a day in my kayak, both places where I use hydration bladders. If I expect to be out longer, I would also expect to plan where I can refill once or twice per day. At that point, having room for an additional liter of water might mean less room for clothes, food, safety equipment, etc. and just doesn't make sense anymore.
It seems that 3L is the balance point between enough water to get you to the next refill opportunity, and not having so much water that it wears you down.
EDIT: The OP mentions situations apparently involving more than a single day hike, and away from any source of water. I assume the hikers must bring all needed water with them and have accounted for this in their weight calculations.
In this case, having multiple moderate sized water containers is far better than a single large one. A bladder is for the water that I am actively drinking via the hose- I can always take a trail-side break and refill the bladder from my own other sources. Having water in multiple containers allows me to balance the weight around within my pack, and not all in one place. Finally- with appreciation to both dualed and Matthew Gauthier in their respective comments- redundant containers prevent you from losing all your precious water to a single leak.
Hydration bladders and dromedaries serve two slightly different purposes,
A single individual probably won't need more than 4 liters and two people aren't going to share one hose.
On the trips I have been on with a dozen people or so, each person has their own 1-2 liters or water bottles/bladders and then we bring three 10 liter droms to bring water from the creek up to camp with.
