Answer :
Because it's their fireteam!!! If you don't like it join a different fireteam or create your own.
Don't sit there and argue or complain about their requirements, they are not going to change it because you don't like it.
My requirement on LFG for doing a raid is always a Grimoire requirement, if you don't have enough Grimoire don't sit there and argue with me or complain that it is a stupid requirements, just go and find a team that wants you.
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I will be posting must have ps4 or xbox one on new raid posts for my team think it will be hard to find a ps3 or xbox 360 team for the rise of iron raid