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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Because I'm a rediculos person.