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.

(Not enough reputation to comment here, so apologies for the "new answer" instead)

This isn't an option in the UI, but is an option in the command line tool.

VBoxManage modifyvm "vmname" --defaultfrontend headless

• Sets the VM named "vmname" to headless start if you just click the big green Start button

VBoxManage modifyvm "vmname" --defaultfrontend separate

• Sets the VM named "vmname" to detachable start if you just click the big green Start button

VBoxManage modifyvm "vmname" --defaultfrontend gui

• Resets the VM named "vmname" to the default integrated (non-detachable) GUI start if you just click the big green Start button