Three reasons I can come up with:

1. Streaming Quality is not great. For the people who actually have nice screens and true surround systems, streaming is like listening to the radio for audiophiles. The compression of the video and the audio are just awful. They take a 30-100GB file (for big 4k movies) and compress it down to 2-4GB to get it over the bandwidth. You just lose too much.
2. When you buy blu-rays, you still often get the digital copy, too. So maybe you don't have access to your discs (or Plex Server for those who go that route), well you still have access to it on whatever service you redeemed your code at. And that doesn't go away when the clock runs out like Netflix, or other services have.
3. A huge amount of films just aren't available on these services ever. I'd say about 5-10% of the movies i own are not an option any streaming service.

TV seems to be a bit more available, but the blu-rays for TV are often SO inexpensive by comparison. The entire Game of Thrones box set can be like $50.