GOG is great. I buy from them myself sometimes. However, it's also an anomaly, and much smaller than the obvious comparison in this case, Steam.
And in terms of things being available immediately, the economics of mainstream movies are quite different. There's a whole world of niche content out there that doesn't generate gazillions of dollars and that won't be leaked when it's barely reached theatres. For smaller operators, deterring casual infringement even to a moderate degree can literally be the difference between making enough money to continue operating and going out of business.
And in terms of things being available immediately, the economics of mainstream movies are quite different. There's a whole world of niche content out there that doesn't generate gazillions of dollars and that won't be leaked when it's barely reached theatres. For smaller operators, deterring casual infringement even to a moderate degree can literally be the difference between making enough money to continue operating and going out of business.