Just click-mount an installation disk image and drag an app icon to the Applicationss folder - isn't this a perfect install UX?

No. Please do set up a usability test in a lab and watch people try.

You will find:

1. Many users struggle with unusual mouse/trackpad movements like drag and drop, right clicking. That's why the Mac theoretically has a one-button mouse.

2. The instructions for what to do differ between DMGs and usually consists of a single arrow if present at all. This isn't clear enough to communicate "drag and drop" to people.

3. If the user figures out or has been shown that they have to drag and drop, they may then be confused when it appears nothing has happened, or by a small dialog that appears and then quickly disappears. It's now not clear how to actually start the app. Nothing appears in the dock, the app itself doesn't start. Launchpad made this better some years back because now there's at least a button to push to show you all apps and let you search for them iOS style, but the user has to realise they haven't started the app.

Note that if you use the App Store there's a sort of animation that (if the user sees it) suggests the app has been deposited into the launchpad. But you don't get that with out-of-store installs.

4. If the user isn't very familiar with this DMG process they may double click the app to start it from the DMG itself, which will look like they successfully installed it (because the app starts) but which may (a) break the app in subtle ways if it expects to be able to write to its own directory, (b) confuse the user when the folder disappears along with the downloaded file button in their browser, thus giving no obvious way to get back to it, except via a realisation that the window which popped up was a folder despite not looking like one and thus could be perhaps relocated via the Finder, unless you rebooted in between in which case maybe not.

Bizarrely and against all rules of good UX design the right thing to do isn't the simplest action that appears to work, but rather, several more steps in between.

5. If they do manage to drag it, find it in Launchpad and start it, very likely they won't realise they're supposed to "eject" the DMG to get rid of the prior copy, even though nothing is actually being physically ejected anywhere. They may also be confused by the presence of two icons that should be equivalent but aren't. If they do know they're meant to eject/unmount it there's nothing obvious to let you do that, for instance there's no button labelled "Eject", but rather you're meant to find the icon for the DMG on the desktop (which is covered), realise it's an icon that represents the window you saw earlier although in the absence of a branded icon there's no indication of that, be mystified by the strange metal object in the icon (who has seen a real HDD these days?) and then realise you're meant to start dragging it again to the trash can, which magically turns into an eject symbol? Or you could try using the Finder, in which case the sidebar entry for the DMG is going to be under "Locations" in a scrollable area that doesn't have scrollbars, and no visual indication it can be scrolled, and the icons next to the name of the app don't indicate what they do, and if your Finder is set to use e.g. tree mode then clicking it shows a view totally different to the one you saw earlier!

The entire UX is something only a UNIX hacker could ever think makes sense. Unless you have a really solid grip on filesystems, nested folders, mount points etc this whole thing is just totally mystifying and a lot of pointless busywork too.

Some Mac apps have code that detects when the user has made these sorts of mistakes and will offer to move the app to /Applications for them. It's intended to partly work around these usability problems, but ultimately, a PKG is still much better especially for videoconf apps where the standard way to start them is via the browser and not by finding an icon in Mission Control.

Windows: let every app you install do anything it wants with all the system files, leaving traces after uninstallation is a norm.

Mac apps leave stuff behind all the time too because they don't have any uninstallation procedure. On Windows it's at least a bug in the uninstaller which could be fixed. On macOS it's a fundamental design issue with the OS itself.