Replying here to a related question in this other thread with topic “Best most powerful zigbee stick?”
https://community.home-assistant.io/t/best-most-powerful-zigbee-stick/262095/
Yeah, black anodised aluminium heat sinks enclosures just look cool when sold in retail stores, haha 
Seriously, if the right parts on PCB board itself has proper RF shielding (which I understand it does now) then that visable metal case is purly aesthetic choice making it easier to sell in brick and mortal stores.
However guess is a metal case probably adds unnessesary cost which is passed along to customers.