Literally go to the other threads about tomestone stuff instead of talking about that here. Not touching it here because this is about RNG, not tomes.
Should every enemy in the game drop every item they can all the time?
Should every chest drop every item it possibly can? This means more than just armour, including cards, music rolls, furnishing items, minions, ect.
RNG has always been a time sink to get people to play any game just a bit longer than if people were just given the items right away. It pads out playtime. This is true for pretty much every single RPG out there, online or offline.
It's also RNG for how much damage you do. No matter your gear you have damage values between X and Y, with other things changing it like Direct and Crit hits. Two people with the same exact gear/stats won't have the exact same damage, and I'm ignoring player skill in this, as well as ignoring debuffs put onto enemies/buffs put on player characters by others.