Competitive should always have been solo-queue. Quickplay can allow groups.
It’s too easy for one player to be boosted by another in competitive with grouping enabled, as well as causing dramatic match imbalances when the extreme end of SRs are grouped (the infamous situation of Silver & Diamond icons in the same match)… no a silver and diamond can’t group, but the groups can be formed and paired in such a way that this is the end result.
That match will never be fair or competitive. 6 Golds don’t equal 3 silver and 3 platinum players.
edit: See the post below for an example from the forums!