Because that's not how it works. It's a Raid, not progression.
There's three real strategies, all have a downside:
"All Need" - Only the role for the gear can need it, nobody can greed on it unless the players with that role pass or aren't present (eg such as an alliance with no DRG's), but because the game doesn't check that you already have it in anything but your inventory, players will still "need" on items they already have for seals/desynth.
"All Greed" - Every player has an equal chance for the item, but the system has to actually check that the player doesn't already have it (presently it only checks that you don't have the item in your inventory, not retainers, saddlebag or glamour dresser.) Downside is that players will "greed" on everything, and may intentionally make it so they can greed on things they already have so they can turn it in for seals/desynth.
"Tokens" - Every player just gets a token for participating, however this removes the RNG aspect of the loot roll, thus you only have to play the content once. So if you don't play it when it's new, you'll never play it again. See Alexander and Coil, even Deltascape now.
The token solution only solves one problem (everyone getting something) while creating another one (raid abandonment when it's no longer new)
The Need/Greed solutions solves the abandonment problem but creates a problem where people will need/greed on gear for reasons other than they can use it. The solution here, and I know people will cry about it, would be to make items obtained from raids not be able to be turned in for seals, and unable to be desynthed. Rather these items need to go to some kind of "raid marketboard" where you can trade like-for-like, at no cost. eg "I want (raid) healer's foot gear, trading (raid) DRG foot gear", this ensures that people can get the items they want by playing the raid, and not by simply buying it off the marketboard.
Though that begs the question, why not just allow the items to be put on the marketboard for gil? Because then people wouldn't farm tomes for the tomes gear, people would farm the best gear and whoever can farm the gear the fastest makes all the money (see mount farms.)
It seems like some mid-compromise is needed between the gear drops and the tokens. However I don't see how it can be done without causing the raid to be abandoned faster. There needs to be an incentive to play old content, and not even Khloe gets people to play old content. They could put the old NM raids into a roulette, and the high-end raids into a separate roulette, where 4 wildcard tokens can be traded for any piece of left-side gear from old content, or 1 token for the right-side gear/belt. Though I imagine this would just propagate the existing problem in the roulettes where the easiest raid/duty just keeps being selected.