It comes to down to picture quality and audio quality. And extras.
Streaming services are getting "Better" but still don't compete with Blu-ray and UHD Blu-ray quality. And if you have a larger screen and/or good speakers (i.e. home cinema), you'll only get the best movie experience from the discs, not to mention all of the extras and bonuses that make a great release. Netflix, etc. does a lot of compression. Even "4k" video, which is possible, must be streamed at a high rate and popping a disc in in much easier. And none of the streamers give you uncompressed high res movie sound (Dolby TrueHD or DTS-HD).
To give you an idea of how much movie-lovers require the highest fidelity. Just read the reviews found at places like this:
http://www.avsforum.com/category/blu-ray-reviews/