If you want to get more abstract with it: waterfall is in two dimensions. The process of starting something and working toward completion can be represented as

If you want to get more abstract with it: waterfall is in two dimensions. The process of starting something and working toward completion can be represented as a waterfall (in 2D) but always and by definition must be represented as a line (in 1D) between two points: start and finish. If you project from 1D to 2D, that linearity will always be there (for completed projects, at least). That extra dimension will introduce additional flavour, and waterfall seems to be a very frequent one.