They're trying to cater to the largest audience. And the majority wants things to be relatively fast, regardless of linearity or complexity (the majority of them would prefer the former). When they added certain mechanics, like a puzzle or a boss mechanic that made it comparatively more difficult (thus lengthening the time they had to be in the dungeon) people complained-- a lot. What more if we get larger dungeons or multiple paths? Party finder specific groups aside, duty finder runs will immediately look for the most convenient/quickest path through the dungeon for the roulette tome bonus.

That said, I'd prefer it if they made the dungeons have, at the very least, a branching path or two. I'm actually surprised there haven't been more complaints about the linearity after FFXIII's release (because everyone and their dogs were bashing it for how linear the maps were, and that you could see how linear it was on the minimap).