This isn’t something that’s going to be added, sorry. The projects screen makes sense in iOS because, as Katherine says, there is no file system on iOS, so you need some way of accessing your project files.
macOS and Windows, however, have file systems, and it is absolutely crucial that users understand those file systems and know where their projects are. If we put a projects screen in Scrivener that showed either a list of projects from certain folders or scanned the disk using Spotlight or something (which would slowdown startup), it would put yet one more remove between the user and the file. That’s not a good thing. Many users already have problems knowing where their files are. A project page in Scrivener would encourage users to think the projects are all managed by and contained in Scrivener, in the same way they are on iOS. (On iOS, remember, if you delete an app, all of its files are deleted too.) It would result in users losing work, because they would think that just copying Scrivener to their new computer would be enough, without realising that they need to copy all of their projects across too.
A projects screen is also far from standard and would go against UI conventions on desktop platforms. iTunes does not have a file system. iTunes is a shoebox app that contains all of the music inside a single iTunes library - the music is not scattered across the disk wherever the user wants it. For document-based programs, it is standard across macOS and Windows to have a “Recents” menu and a file open browser, leaving it up to the user where to store files - the same as Scrivener works (because Scrivener uses standard desktop conventions).
On iOS, every single app is essentially a shoebox app, because the files are all placed within a single container dedicated to that app. So apps that deal with files need their own browser for that (Scrivener’s project screen). A projects screen in Scrivener on the desktop would be misleading, making users think that Scrivener was managing all of their files when it was not. The only way it could work would be to remove the ability for users to save projects anywhere and turn Scrivener into a shoebox app that automatically stored all your projects in a single, opaque library, and I really wouldn’t want that.
All the best,
Keith