While it wasn’t the answer @yearofglad was hoping for, I think we’ve more or less answered the OP, so I’ll feel free to continue the sidejack.
@Thudlow_Boink, I believe your assertation is correct, in that the pricing is largely set by the publisher these days - when ebooks were a niche thing, well, there was a lot of flex and freedom, which got locked down the more successful they got, kind of like we saw on Netflix’s streaming service which was once a sidebar to the discs, then a standalone service, then the majority of the business, before the vast splintering began.
Is an …accessible article, although I find it a bit apologetic overall, but lays out most of the main points. It brings up points we mentioned earlier: ease/speed of access and agency pricing set by publishers, but brings up another we haven’t mentioned.
Almost all the digital publishers are arms/branches of traditional publishers - and they don’t want to sacrifice one side of the business for the others. If you price the ebook too attractively, you kill your paperback/hardcover market. And I’m sure that applies to music and movies as well - if you kill your in theatre sales (well, prior to COVID) or whole album sales with inexpensive online offerings, you’re really leaving money on the table as things stand.
Back to the DRM issue though. IMHO (and sorry, I know this is FQ) it’s part of the backlash from the years of pirating. I just don’t think it’s a rational one. The golden age of music and movie piracy being bold and brazen is long gone, but it’s just as prevalent, and only somewhat hidden. The various publishers don’t want to make it even easier, and DRM is a tool to discourage even the less malign versions. Back to the OP for an example, why let the licensees of a DRM free movie share it with their friends via PLEX, when there’s a chance said friend would rent/buy a copy of their own. Again, from their POV, that’s money lost.
And that’s just an individual, there are/were plenty of people who would put up digital copies of films on youtube, pirate sites that constantly move, or sites that are otherwise largely out of reach of their armies of lawyers.
It’s freaking frustrating to the private user who wants to consolidate access to their digital content, but it’s understandable.
What really irks me though, is that every once in a while, a digital carrier will experiment or embrace a virtual sharing system but it gets immediately crippled by cross platform or ‘eligibility’ requirements. Again, no money to be made by those publishers, the same ones years ago who wanted used bookstores dead with extreme prejudice.
So long story short, I’m betting that while the DRM based services are indeed leaving a bunch of @puzzlegal’s money on the table, they figure by using DRM to stop/slow piracy, they believe they’re keeping a lot more of the public on the whole’s $$$.
ETA - and we’re also leaving out other factors, like Amazon selling it’s Kindle tablet lines fundamentally at cost, assuming that the captive digital sale of books/music/movies/games on those devices will quickly recoup those losses and become profits in short order.