USB passthrough of a hub

[moozer]

I saw this reply

Post in thread 'Passthrough of USB Hub that supports Per Port Power Switching'

Apr 27, 2023

Unfortunately, you cannot passthrough whole hubs at once. You need to USB passthrough each port (or device) on each hub separately.

• leesteken

ok, we cannot take a usb hub and expose it to a vm, and then plug in usb devices without doing things in the virtualization host.

My question is where this limitation comes from.
Is it a hardware issue? is it qemu?

Any links or pointers would be appreciated.