I mean. Because it's testing?
People wear and do different things in beta/PTR to see if things work as intended, see if things are broken, and see if there are exploits that devs should catch.
This isn't a preview of a raid fight. It isn't a practice round to make sure the mechanics are nailed down before it goes live. I don't know why anyone wouldn't understand that bugs need to be caught in testing phases, by any means available to the player.