> When I delete an app on an iPhone, all its data is also deleted. Gone. Why can't it be the same for the Mac?
Because on iPhone the app is completely sandboxed. Think Docker, with virtualised OS paths, but on OS level. When Apple tried to introduce a very light version of sandboxing, the world was ablaze in pitchforks and torches.
When I delete an app on an iPhone, all its data is also deleted. Gone.
Nope. Delete your Google apps. Reinstall one. It'll still pick up the accounts you used prior. Well, it used to anyways. It's not an iCloud keychain thing (or it wasn't).
There is an exception on iOS as well. Apps can register write access to a custom location on iCloud that will persist after the app has been deleted. You can see all the leftovers in Files.app
That's what the average end-user understands by removing an app from the system nowadays.