The idea is to just have a very quick 1 click way to find scripts that you have, Google Drive is more of a general purpose storage, this is specifically for scripts and optimized for that.
To list a situation where Waterdeep could be easier to use:
When you have a temporary file you might need but is messing up your git flow, you can just store it at Waterdeep and access it later or delete it. If you want to do the same with Google Drive, you'd have to create that file inside a folder which can be hard to find.
For the case of GitHub, you can easily have your own scripts there and it would work fine, Waterdeep is more so focused on extremely fast access to your scripts without having to create a repository and commit/push the files to it, which I understand it pretty straight forward to do for most developers yet that friction might be the reason why most of them don't have a dotfiles repository on their GitHub account.