Apple - Why are so many Mac downloads a .dmg just that contains a single .pkg?

Technically, a .pkg is not a file, but a folder. So maybe collecting it into a single file is essential for downloading.

Data on DMG image can be checked for integrity with Disk utility. You can make it mandatory for the user to agree to license before mounting dmg. It could be locked from modification. You can easily add hidden files and folders on DMG. You can make background image. You can make it safe from stripping during transit through non-metadata aware file systems like FAT.

Plenty of reasons actually.

One possible reason could be that Safari by default automatically mounts DMG files, and that a Finder window would pop up with the contents. In turn the DMG folder could be styled with for example a background image. I assume that could be considered more user-friendly than having to look in your Downloads folder for an installer, but I agree it's pretty moot.

Another reason would be the ability to group together other supporting files as well, for example documentation files or even secondary installers.

However, it's not such a common distribution to start with. The majority of apps are distributed as self-contained .app folders inside a DMG file (if not via the App Store now). Your example seems to be rather an exception to me, though maybe it is more common with some particular distribution channels? Personally, I wouldn't dwell on it much further.