Completely different products for completely different use cases.
Google Cloud Storage is a storage on cloud, no more abstractions. If you want to build a document management system from scratch, you can prefer it as the storage provider.
If you build an app on the top of Google Drive, you inherit a file system abstraction, user management, a permissions model and etc. But you don't own the users, neither their drives. Additionally, Drive's quota management is fined tuned for "per-user" usage. Most people think creating a single Drive account and logically share it among their users on the application level will work. It's unlikely to scale due to the quota limitations.