Some are intentional, some aren't. Among the intentional, some are intentionally trying to make you wait, and some aren't thinking about, or even knowing that, you'll wait extra. Any change to penalize DCs will penalize some unintentional, some intentionally hitting a button but non-toxic, and some intentionally toxic.
Punishing the unintentional for something outside their control, for something unrelated to their skill, is pretty unfortunate. Being penalized in your skill score for something outside your control is also anti-fun. If we're going to punish them, we'd like a pretty big reward to make up for it.
We're historically not convinced, and I'm presently not convinced, that the reward is worth it. How much does the user experience actually gain? What percentage of games end in an intentional leave-without-forfeit? Definitely a minority of them. Among these, not everyone would be dissuaded by the penalty, anyway. Some people will still rage leave despite a penalty. (In my experience, the better players who care about their skill score more are less likely to leave without forfeiting.) And the time waste just isn't that large in the minority of games it's relevant. Especially since, if the game is so over in your favor that the other person is leaving, it's often easy to start another match while you're waiting. In the unlikely event your opponent comes back, it's often pretty easy to wrap up the game, even while you're playing another one.