One of the factors contributing to it not necessarily being cheaper to buy box sets: Target, Best Buy, and other big US chains have given up on physical media for movies. Last I heard Wal-Mart is on the fence about long term support for it. (At the moment the only chain I know that seems still in it for the long haul seems to be Barnes & Noble, but they could have one bad quarter and change their minds quickly. My local store the DVD and Blu-Ray section is somewhat obviously what used to the Music section and still has a bit left of that old focus.)
Even the manufacturing side seems to be disappearing. Sony has flirted with dropping physical media sales of movies. Warner Brothers and Paramount seem likelier to drop physical media sales if they are allowed to merge.
Netflix lets very few originals make it to physical media. Most of the exceptions I'm aware of are Sony projects. Sony Animation being one of Netflix's last exclusive deals from a studio that isn't themselves. A lot of Prime Originals don't make it to physical media. It seems a mixed bag for HBO Originals and Hulu/Disney+ Originals, presumably based on some internal formulas of popularity. A lot of Apple TV Originals seem to get delayed physical output, but that's largely been because Apple TV doesn't have its own studio, but that changed relatively recently with the purchase of Red Hour (Ben Stiller's studio, of Severance and The Dink fame).
It certainly feels like physical media is dying right now. Maybe more reason to switch to investing in it over streaming services, not that an individual's budget decisions in this vein will affect the company's bottom lines enough that they will notice.
I've come across sealed Blurays or even the occasional former Blockbuster rental so it's actually kind of fun in a record-crate-diving sort of way, if you're into that. But of course you can always just buy new, which you'll probably have to do in some cases: the odds are against me finding Koyaanisqatsi at the thrift, let alone the Criterion Collection Qatsi Trilogy, but I'm okay with that.
Have to take things into your own hands these days and grab the 2160p Dolby Vision streams and stuff them onto a BD-R, if you know what I mean.