Or is it because they used critical thinking and thought through what those "punishment" systems would likely cause as a by-product?
Consider the various scenarios:
Currently, Bob doesn't like the run (for whatever reason) and leaves. Yeah the group "wasted" X amount of time and the key is gone. Individual members go find another group.
System A: Punishes first player that leaves or DC. Bob doesn't like the run but doesn't want the punishment. Now Bob continues to "waste" the rest of the groups time by making DAMN sure it's going to go as long as possible until someone else leaves and Bob can now leave without a negative mark on his account. So instead of just wasting X time, you're now looking at X+Y time but now it involves everyone.
System B: Uses some form of voting to either abandon or stop the run. Again if Bob doesn't like the run AND he's in the minority then he can still make things more difficult until another vote is held.
System C: Some form of being able to replace a member that leaves. Now instead of worrying about leavers, you get a different issue that M+ turns into a carry fest. Do the dungeon with the first team but save the last 1% (or less) of trash left, switch out members for the carries then finish.
Still haven't seen any good robust "punishment" system that has automated safeguards against exploiting or gaming the "punishment" system.
And as a counterpoint, Blizzard HAS provided "carrot" incentives to stick around. Going slightly over time (less that 40% of timer) still gives points towards score. Loot still drops at End of Dungeon chest (albeit one will have lower ilv) and finishing dungeon still counts for weekly Great Vault.