Content is not fungible. When I want to watch Terminator (1984) and nothing else, then no amount of having other killer-robot films on your platform will satisfy this want.

As to why people who buy prefer to buy physical? Because there's actual ownership. I haven't even as much as investigated my options for buying 4K content, because I am 100% sure it doesn't exist in a form that's not hooked into some DRM system (AKA worse than useless). With physical content, at least you retain the control.