But guys, what if his cat really WAS on fire?

But seriously. This would cause much more harm than good. How often do you run into a 3/4 or 7/8 premade that acts like a group of jerks, compared to running into 1 guy that acts like a jerk (and would leave if there wasn't a penalty because his time is too precious). If you have to go because of a real life issue, its not like the lockout is harming you. And if you have to do something with your FC, being in a full premade bypasses the penalty.

And do you really want to overwhelm the GMs with (more) countless reports of someone leaving a dungeon for no reason? Additionally, if they did decide to impose a penalty on a case by case basis, the penalty would likely be huge.