There's a lot of AAA games I havent tried, but why are the launch day patches Ive experienced so gigantic?

Why do games need to download textures, sounds, videos over again? I just can't understand why a 100GB game has a 60GB patch etc, other than purposeful obfuscation or complete disregard for care.

Ironically, frequently _because of_ compression.

AIUI the process of building asset packs and compressing them can mean that modifying a single texture can mean complete reshuffling of the resulting file; and that means redownloading the whole compiled "pack".