Curious: For most modern Display Smartglasses, why did they often need a secondary device to simply move the display to the side of your view

I been wondering this for a while. But why do most modern display smartglasses needed a secondary device simply to move the display window to the side of your field of view?

Xreal had the Beam to do this (which their newest Smartglasses wont need anymore)

Viture had the Neckband to do this.

But why the need for a secondary device for this? Why couldnt past display glasses already do this? just curious.