> Is the whole thing just so that the file is DRM-ed to hard that nothing else except expensive equipment can play it?
Could be the case. However, one of the quirks about Digital Cinema Packages (DCPs) is that they don’t use video codecs like H.264/AVC or H.265/HEVC. Instead, to maintain maximum quality, every single frame is individually compressed and stored with JPEG 2000, and they are played through like a slideshow, just at 24 frames per second. Needless to say this adds compute and especially storage concerns. Storing every frame of a movie individually quickly hits hundreds of gigabytes if not terabytes; and you need to decode and have every frame ready in less than 41 milliseconds, which is shockingly difficult even on high-end hardware. Even a modern Core i9 would have to be parallel and have multiple frames ready at a time because doing it a single frame at a time would be too slow (and thus, most projectors almost certainly use custom chips or FPGA-based decoders but that's really, really expensive). Do I need to mention read speeds and network traffic? If every frame is, say, just 5MB, that is over 960 Megabits per second, saturating a Gigabit link.
Another thing to consider is the multitude of speakers. Dolby Atmos is object-based, meaning there aren’t specific audio streams for specific speakers like in the past. Every speaker’s location is individually stored in the playback device varying between theaters, and the sound is calculated and mixed for that specific theater’s audio setup at the time the movie is played. When you are calculating audio in real-time for 30 or more speakers, the compute costs add up, potentially into custom hardware / FPGA territory. Also that "saturating a Gigabit connection" doesn't include the amount of data that the audio uses, and considering it's 24-bit PCM without any compression for dozens of streams for the objects, that's also pretty substantial. Just 2 streams of 16-bit PCM for ~70 minutes on a CD takes about 700MB.
So, just for one movie, with every frame taking 5MB (possibly more or less, we don't know how big they actually are) and audio taking whatever it takes, you would need a 2.5Gig Ethernet link to be safe, and storage that can deliver that (which is also why an external hard drive is out of the question for actual playback, only very recently do we have built-in NVMe SSDs that can deliver that kind of speed, but we still don't have any external SSDs that can deliver anywhere close; it just has to be RAID on multiple storage devices). Now consider what you need for a theater with 12-20 screens at a time. The storage amount and speed requirements is substantial. Now consider this has been standard for over a decade and think about how expensive this would've been to implement in, say, 2013. Oh, and that storage had better not fail, ever, or your entire theater is down, and there's a lot of refund requests and unsold tickets until it's running again.