Why haven't they gone digital?
However, IMAX tried to go mass-market around 2008 and that's why you see so many supposedly-IMAX theaters now. They typically use 2K DLP projectors. Yeah, basically the same res as a gaming PC from 2011.
My theater has the crummy 2k digital, and let em tell you, you can DEFINITELY see the pixels. I would call it passable but it's not really great at all. The only benefit of these IMAX locations is that normal digital cinema can be even worse!
There are newer 4k laser projectors that some theaters are retrofitting into their old huge IMAX locations, but these still aren't going to hold a candle. 4k still isn't even enough to really come close to 35mm.
The digital tech just isn't there yet.
EDIT: Forgot to factor frame rate into the math. Divide that by 24 frames per second. That got me curious, so I looked into it and found they're using JPEG 2000 on each frame with no inter-frame deltas. Essentially like a constant stream of I-frames.
That has a photographic area of 864 square millimeters.
70mm movie film has 3395 square millimeters. That's like 943MP - 1886MP digital equivalent, if we use the same standards as 135 photographic film frame.
There's no way you can replicate that amount of quality with digital cinema, at least right now.
IMAX is similar in size to 6x7 medium format photography, which only recently has been fully replaced by digital cameras. The cameras that replace medium format photography (usually used for stuff like magazine cover photos, high end product photography, high end portraits and other high quality still applications) range from the sony a7r4 with a 61MP sensor, hasselblad cameras with 100mp sensors and phase one with a 150MP sensor.
The digital video equivalent is 16K, which exists but it's just not common at all.
To be fair, he recently said that 70mm IMAX format is also ideal as master for any downstream format, because all the information you're ever going to have for that movie is in the negative. And with the length (and therefore the size) of the movie, he acknowledges he may at last have reached the limits of analog film technology.
He is like that in interviews, but it's worth noting that a lot of care is put into making great digital versions of his movies too. I'm not familiar with how involved he is in that process, but I would hazard a guess he's pretty involved.
The Oppenheimer site mentions this:
The Digital Cinema presentation of OPPENHEIMER has been created from 8k scans of the photochemically color-graded film elements, scaled to 4K, fine-tuned in the digital realm to maximize the color and contrast attributes of digital projectors, and dust-busted to achieve the cleanest and most stable image presentation possible. The film was finished in 4k for the highest digital resolution currently available.
I find the audio mix to be horrible in his other films. Like, worst I've ever heard from a movie with a budget like he has.