Because sometimes you want to view a particular film regardless of whether it's still available on your favourite streaming service.
4k Blu-ray's are higher bitrate than anything available to stream, and still not everyone's connection can even cope with 1080p streaming.
Also useful when Netflix randomly decides to not stream at anything better than 240p or your internet craps out completely.
+1
I can buy an old release DVD for a couple of pound to own it. I can watch it when I want and how many times I want rather than paying £5 a time to some streaming service to just "rent it" once.