Why can't I queue with my platinum friend but I can get matched against plat players?

Scenario is as follows. I play with a 5 man squad from time to time and we want to play flex. 2 of those people have recently entered plat in flex. the others in silver and gold. Now when we want to play flex the 2 in silver can't queue with the plat one because they are to far apart. So those 2 went on to play flex and they get matched against plat players. Why the %%%% can't they play with plat but matchmaking thinks it's okay to put them against plat?
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