I buy them. Streaming services encode streams using lossy compression for both sound and video, and the difference is noticeable, especially for 4k movies with HDR and TrueHD/Atmos. If you have a decent home theater setup, the discs are worth it.

Streaming services' bitrates top out around 40 Mbps (4k UHD on Apple TV), and most services are actually closer to 20 for 4k, often even lower. 4k UHD Blu-Rays have up to 128 Mbps bitrate. The quality difference is evident, including in the sound. You can see compression artifacts in dark scenes on streaming, and jagged lines in color gradients. The simulataneous dynamic range of sound (some things being loud while other, quieter things are audible on other speakers at the same time) is lower on streaming. And on top of all that, the quality is inconsistent, and the stream will drop into a lower resolution for no apparent reason.

I get the sense that the market is drying up, but I hope it doesn't, because the quality of streaming is legitimately worse. The discs are currently the only way to get uncompressed quality, unless I guess if you find a torrent with a rip. But that would be a 50-100 GB download, for one movie.

Also, when I buy the disc, I own it. It's mine forever, in its definitive edition. No scene cuts/edits for political or monetary reasons are possible, and streaming services cannot yank the movie and prevent me from watching it.