Googler opinions are my own. I've never worked on Drive. It's not hard, but expensive. If you have a file system where it is a proper tree in any given node w

Googler opinions are my own. I've never worked on Drive. It's not hard, but expensive. If you have a file system where it is a proper tree in any given node within that tree only has One parent, and walking up all the parents you always only have one parent up to the root node, you gain some useful properties from that. You can build a permission model where you just have to check the permissions of all of your parents to see if someone has access to a given file deep within the system. This means that if you Grant Reed access on any node within that line of parents, all children implicitly get access. Now imagine a directed graph, and you set permission on a node and you want to say all connected nodes also should get this permission. You could definitely walk all the nodes and apply that permission, but now you have to write said permission to every node, rather than being able to set it on a parent and let all children inherit it. Even if you know all of your parents, then all of those nodes have multiple parents. And you can have cycles. Checking parents for permissions becomes way more complicated for users to understand, which is why Drive likely didn't do this.