Why doesnt write behind caching solve the "many files slow copy" issue?

Question

Today I was copying a node.js folder to another place on the file system, and I was wondering.. with SSD and write behind caching, why is the copy many files problem not purely a case of "figure out all the changes in memory and then commit the bytes to disk"?

I know there are many steps to copying a file including making space for it, transferring the bytes, adding directory entries etc, but in terms of storage performance it should at least theoretically be possible to perform much of that work upfront, instead of one by one synchronously in the file system.