The iPhone dispensed with the notion of a user accessible file system. You no longer "saved" a document, or had directories or folders to organize data. All data for an application is managed internally by the application. You can only access a document using the respective app (*).
The result of that is that nowadays most people are sadly oblivious to the notion of a hierarchical file system.
The iPad when introduced used iOS, and thus was based on the same system (iPadOS is but a variation of iOS to this day, with some extra features to make some use of the larger screen, but essentially the same).
So when you delete an app in iPhone or iPad, all the data goes with it.
That is by design.
The Mac, however, does offer a user accessible file system. Documents are separate from applications, and you can save files in different locations, create folders, easily use external drives, and more.
Furthermore, there are separate user accounts, so you can have different users on the same mac, and each have their own content, completely separate and private; that why you can have your Documents, Music, Movies, Pictures and other folders, and organize your content in subfolders, etc.
On the Mac, deleting an application does nothing to the files it operated on, because they are totally separate. A FCP library is a "document", like a Word or PDF. You can save it in different places, copy it to an external drive, and even look inside it (don't, unless you know what you are doing).
The iPhone effect makes it so that some users don't even know where their libraries are stored...
(*) This has changed somewhat due to the need for interoperability (iCloud, Handoff, etc), but still the Files app on iPhone and iPad, even though it lets one create folders and organize files to some extent, is limited by the general philosophy of the OS it lives in. Still, I'm grateful that it exists.
