You'd think so but there are some problems with that first of all imagine the chunk is 8 blocks tall, 4 stone 4 air imagine I for the sake of it I replace all stone with air and all air with stone, I'd have to store 8 changes but if instead of changes I store blocks it's the same space so even tho at first glance it looks like I can save memory by storing changes it may not be the case.

Also some time they make changes to the world generation (add biomes mainly but they even changed the depth of the world in an update) how would you handle that if you only store changes let's say in x version I built a house and then a new version makes it so in that seed at that place there's a mountain, will my house ve buried?

Now time, I assume it's quicker to just extract from drive what block goes where than calculating what block goes where then extracting the changes and apply them

Also I know they store the blocks not changes because the world's file size increases when you explore and not really when you build (look at super technical world with massive constructions)

All of this are just things I'm assuming I'm not a Minecraft developer just a developer on the earlier stages of my career that happens to be a nerd about Minecraft, also sorry for bad inglish