That is still a holdover from television: predictable equally-time episodes. Why should each episode be of identical length? Why not free the production to tell each episode in a different time. An even more free form would be, for example, 16-hour non-episodic movie where the viewer is left to decide when to take breaks.
And if you release something all together in 16 hours it became harder to go back to where you stopped, in case the software glitches, harder to comment with friends, which maybe would create less hype, and maybe even create a mental barrier, like, "omg, this is 16 hours long in one go!"
And how are you even supposed to know when you should take a break while watching it for the first time?