> It is solely so that people can include a shortcut to /Applications and style the dmg window background with a bitmap that alludes to dragging to install?
I believe so. The first app I checked, DaisyDisk, distributes as a zip file. Like you said, most often people distribute dmg and it's to give hints to drag it into /Applications. I really like the .app system, but always found the UX of download, unzip/open DMG, drag to /Applications as very awkward. I figure Apple stopped caring about that when they added the Mac App Store in 2011.
App bundles don't use resource forks, but they do often use extended attributes and aliases and things. Although it's possible to make a zip file with those things, not every zip program does it by default.
But mostly I think it's just tradition, and maybe some small secondary features like being able to show a EULA when someone double-clicks the disk image.
It is solely so that people can include a shortcut to /Applications and style the dmg window background with a bitmap that alludes to dragging to install?
I know that at least hfs+ still supported resource forks, but I don't think any modern .app bundle has any anymore, or has for years. (Does APFS even still support storing a resource fork?)