The main reason CG couldn’t make the change you suggest is because of the imbalance issues it would immediately create.
There is essentially one way CG could implement a flat reward system and it’s exactly how you described what happened with the Sith Raid. For everyone to get equal rewards in the guild, the overall rewards given out would need to be watered down considerably.
It’s a matter of deciding: would you rather have a chance at getting top 5 rewards or just get a flat 30th place (just an estimate) rewards every time?
Personally, I always seems to get fully crafted gear when I place outside of the top 25 and get garbage when I’m in the top 5