Because that works? The big problem if you have multiple persons working on the same zone at the same time is that you have to spend quite a lot of time stitching the entire zone back together. Every time you rework a bit of the zone (and it usually happens a few times), you have to spend a lot of time redoing the stitching.
If, on the other hand, you have one person doing the zone, then it takes longer to make the zone, but you use less man-hours. Modern tools probably make that easier, but in the early 2000s, that worked well for a number of teams using 3DS or Maya. That's how we did it with Ryzom. You had a separate group doing props and yet another doing spawns, but the base zone mesh was one zone per person.
(it would be different for a seamless game like AC1, but AC1 had heightmap-based zones, so stitching was not only absolutely trivial, but the base zone itself was extremely fast)