Mainly it's because they're lazy. They don't want to have to do all of the work of pulling a DVD off a shelf, inserting it into a player, skipping through trailers and promos, and navigating through menus to get to what they want to watch.
But even streaming has drawbacks too, namely. . . .
This is the biggest complaint I hear from people who have Netflix is when shows and movies are removed from their service, then it's like end of the world. One-time purchase of the DVD and it's yours forever, whenever you want to watch it, without having to worry about it being taken away from you. So what if you have to do some physical activity of having to put the disc into the player and navigate through some menus? Once upon a time, people had to get up out of their seats, walk over to the TV, and turn a bunch of dials to change the channel, adjust the brightness/contrast, change the volume, etc. because remotes didn't exist then.