Because GS is NOT a sign of skill or experience. Just that peeps got gear... usually thru being kited by a handful of skilled geared peeps.
Here is a recountof a recent ICC 25 pug.. WITH 25% raid buffs. I was almost denied a spot, but they couldn;t get the spot filled so I was in.
I am the Shammy at the top. In the first 4 fights.. the meters looked like this... I was in the top 3-4 everytime.. and mostly top 1 or 2. This was my entry level geared alt.. main is a mage that is 6k gs. I know the fights and I researched my toon.
I was the LOWEST gearscore in the raid... and mostly by a large margin... (ingame GS is about 2X what you see.. so I has 400-1000 lower than the rest.
All I see in trade is "Must have 5500 GS or better" all this is are peeps who don't know JACK about the game relying on a mythical figure from a lame azz addon to get people to kite them thru raids. What they get is mainly other peeps who have BEEN kited to get better gear.
People have come to rely WAYYYYY to much on GS... and they STILL aren't capable of downing anything requiring a little situational awareness. I'm not impressed with my own marginal performance in that 25% buffed raid... but was a FAR cry ahead of MANY MUCH better geared peeps.
Yeah.. Gearscore was used for this 25 man pug... and the skill level prevented us from getting past first 4. Plain and simple.. Gearscore is used by peeps who really don't KNOW what they are doing.