Helldiver's engine does have that capability, where bundle patches only include modified files and markers for deleted files. However, the problem with that, and likely the reason Arrowhead doesn't use it, is the lack of a process on the target device to stitch them together. Instead, patch files just sit next to the original file. So the trade-off for smaller downloads is a continuously increasing size on disk.

Generally "small patches" and "well-compressed assets" are on either end of a trade-off spectrum.

More compression means large change amplification and less delta-friendly changes.

More delta-friendly asset storage means storing assets in smaller units with less compression potential.

In theory, you could have the devs ship unpacked assets, then make the Steam client be responsible for packing after install, unpacking pre-patch, and then repacking game assets post-patch, but this basically gets you the worst of all worlds in terms of actual wall clock time to patch, and it'd be heavily constraining for developers.

from my understanding of the technique youre wrong despite being 80% sure ;)

any changes to the code or textures will need the same preprocessing done. large patch size is basically 1% of changes + 99% all the preprocessed data for this optimization

It goes all the way back to tapes, was still important for CDs, and still thought relevant for HDDs.

Basically you can get much better read performance if you can read everything sequentially and you want to avoid random access at all costs. So you can basically "hydrate" the loading patterns for each state, storing the bytes in order as they're loaded from the game. The only point it makes things slower is once, on download/install.

Of course the whole excercise is pointless if the game is installed to an HDD only because of its bigger size and would otherwise be on an nvme ssd... And with still affordable 2TB nvme drives it doesn't make as much sense anymore.