Correct me if I'm wrong, but you are making 2 assumptions here:
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One often wants to have a Plone website where both frontends are active at the same time for the same content
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That such a setup shoud be a supported in a 'no technical knowledge' needed profile for non technical users/integrators because it is a common use case.
You * don't * want to activate 2 competing editing frontends that are based on different frontend technologies and support vastly different underlying html/javascript outputs for the pages in the same server processes and on the same content.
From a migration/upgrade point of view it could be part of the desire to 'slowly' migrate your site from a ClassicUI frontend to a Volto frontend. And if you really want to, it is indeed technically possible. But you need a lot of experience, really know what you're doing and have very very good reasons to do this and a desire to shoot yourself in the feet. It will take you days to weeks to months to correctly set up, and months and years of future maintenance and update issues.
If you want to have folderish types in classic UI: there's an add'on for that: collective.folderishtypes ยท PyPI . Might need some updates for Plone 6, latest classifier is for Plone 5.2.
The Volto frontend doesn't smuggle a package in the backend. It is by design that you use either Volto, or ClassicUI. And if you want a mix: it's all technically possible, but if you are asking here why it isn't configured for you by default this way or 'made easy', then it is unfortunately unlikely that it is a good setup for you.