Steinberg converted its plugin format to use the "bundle" (this is explained in my FX plugin book).
This matches the way that AAX and AU are distributed. It also allows the plugin to have a known outer folder hierarchy, with the ability for you to store run-time information in the Resources folder (I am doing this specifically for plugins for the new Synth book). For MacOS, the GUI is loaded dynamically from the XML file, which is not compiled into the code as with the Windows version, so it also needs to be in a specific location. This is also why you can use the VSTGUI4 editor in MacOS, make changes, and have them appear instantly, whereas you need to recompile the plugin in Windows to see the GUI edits.
The folder "bundle" does not cause issues with hosts.
If you need more information, you can ask the engineers who made the decision:
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