Among others there's the issue that patching may be a seek-heavy operation. Downloading a 40GB file over a fiber connection is much faster than merging 8GB of new textures into a 32G file on a console that has a HDD.

When downloading from a fiber connection to a PS4 HDD the bottleneck is likely the HDD's speed. To merge a file you would need to at least read it and write it again, which is at least twice as slow as simply downloading it. Alternatively you can pay the cost by seeking constantly instead (like a texture per file), which may be even slower and probably sucks at runtime.