Time and effort and cost (assets, etc.) and the story all factor in. Some stories are stronger when told in a straight/linear fashion.
The term laziness here upsets me for some reason (nothing personal). Might be because I'm a programmer and I understand what it means to need to cut content or features or avoid feature-creep in order to meet budgets and schedules. It's not like "branching paths" are on a standard RPG checklist and developers are just "naw, fuck that".
That being said, there are still a lot of very nonlinear RPGs around. The Fallout series and the Elder Scrolls games come to mind, and I think the Witcher series offers a lot of choice too.