The simple answer to this question is that in any queue content, blizzard is responsible for forming the group, therefore blizzard is responsible for ensuring the success of the group (This is also why queue content has things like luck of the draw or determination, in addition to be hilariously undertuned)
In manually formed groups (M+, N/H/M raids, Rated BGs), blizzard plays no part in forming the group, therefore has no reason to take responsibility for anything that happens in that group.
If people are leaving your M+ dungeon, the blame goes to everyone in the party (mostly evenly, but some extra goes to the leader/inviter), for not doing anything to set expectations up front.