Why is Virtualbox start mode not detachable by default

You can read why in the current version of the user manual for the startvm command:

Starts a VM with detachable UI (technically it is a headless VM with user interface in a separate process). This is an experimental feature as it lacks certain functionality at the moment (e.g. 3D acceleration will not work).

So, yes, this is still a new feature and it is missing some of the features a fully attached GUI provides.