Here's my thoughts on this, as many have said it's something to do to keep people running the content. Sure you don't want to run something 20 times and not get anything, but let me ask you this, are you playing completely solo? One of the things you find a FC being very useful for is having a full set team so that when the item drops that you want you have a set number of people to talk it over with and they can either Greed or Pass while you Need. Or at the very least you know that the amount of competition is down. I personally come from WoW's golden age of having to fight 39 other players for some loot cause it would drop something everyone could use, or at least 5-10 others of the same class in the full 40 man raid. Even when it got up to the 10 man and 25 man raids of WotLK it was still hard to get gear to drop and gear up so you can move onto the next tier of Raid content. I would go weeks without getting some certain items and having to pass on others I had gotten already, or courteously let another get it as they were Main Tank or it was a final piece they needed. Raiding in WoW didn't mean getting 9 or 24 other random people you don't know together, it meant organizing the event with your Guild and working on it week after week as you only had a single lockout per character. So if you went in and downed the first boss on the first night of raiding and wiped on the second till everyone called it a night, you went back in and started from the second boss and didn't get another run of the first one until the weekly reset.

So to the ones talking about making it a lockout for 1 a week, that is a bit of a stretch seeing as how many people want to get this gear. Leaving it on just RNG does mean that you can be at it for 20 to 30 times a week and still not get it, but that's part of an MMO. If you want instant gratification and 100% drop rates, go play a solo RPG where you aren't reliant on anyone else to get it and so long as you beat the boss you get the drop you want, or only have to farm it a bit if it still is an RNG drop.