We aren't talking about the initial downloads though. We are talking about updates. I am like 80% sure you should be able to send what changed without sending the whole game as if you were downloading it for the first time.
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.
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