This is one of those things, similar to PvP vendors, that Blizzard will NEVER be transparent about. They dodge questions about this topic in such cringey fashion during the Q & A’s.
You know they can’t give the real answer on why this crap exists because the real answer is something akin to, “We’ve used analytics and believe these systems manipulate the player into having longer playtime and thus paying us more money in the long term. Because the player must rely on RNG and never be able to achieve BiS without hoping for odds similar to winning the lottery.”
Whereas in the past you could raid and have about a 5-8% chance of receiving a high end or BiS item from the raid, or you had Valor Points and PvP vendors making the rewards more predictable and plan-out-able. Contrast that with today where PvP gear is just existing in some dubious pre-planning force gear-feed even if you out-ilvl a piece and don’t want it way. BiS for PvE requires getting a Titanforge with a 0.00008% item drop rate.
It’s just garbage. It’s just stuff meant to extend your time playing the game, not make it more fun or better.