Perhaps you need to look at the Roku as a one-time subscription to a lot of cool content and the streamer being free. This versus buying some other manner of streamer for so many dollars and having to go out and dig up your own content.Because I already have plenty of computers (and generic media streamers), I don't need any more devices.
Also, buying proprietary systems encourages people to keep making proprietary systems.
Then there's the Apple TV model where you buy the streamer and have to turn around and buy all the content -- piece by piece.