The flash memory isn't fast enough to store all of that in the ~2 seconds it is collected, nor is the RAM large enough to store it all.
The iPhone is literally incapable of taking true RAW photos (at least not if you want to collect all the same info that the built in camera does)
iPhone's hardware is more than adequate to capture RAW videos at 60fps with binning, not just photos (too bad Apple doesn't allow it, so it can't). People are using Android smartphones to shoot videos in RAW for several years already. OnePlus 8 Pro and later can easily capture a RAW stream in 4K 60fps.
The stacking also allows things in the frame to move - it uses optical flow to track movement.
That's how iPhone manages to take a tiny very noisy sensor and give non-noisy output.
If you want to see how the camera performs without this stacking, try taking a photo from a moving car of something behind railings. The moving railings screw up the optical flow, so most of the 127 other frames will be discarded (if regions of an image arent a close enough match, then the region will be discarded), and you'll get to see all the sensor noise.
And all this has nothing to do with manual astrophotography, where you ditch computational apps, use a mount and shoot actual unstacked raw sensor frames with several seconds exposure each, interleaving them with dark frames, and do the computation yourself with the dedicated astrophoto software which is far better than any app if you calibrate the sequence and do the math correctly. Every phone is capable of this.
You still need a third party camera app to capture true RAW photos.
E.g. with ProRAW you can't remove most of the noise reduction, and you can't rescue images with large white balance issues (you get fringing in one of the processing steps since the white balance is baked into several of the intermediate images)