It's to create a sense of urgency, they give you more than enough time. If healers/DPS/etc are pulling ahead of the tank then it just means they are bad players, not afraid of a time out. Not everyone has 8hrs a day to play so they don't really want to spend 2hrs in a low level dungeon they had to wait 30min to even get into. Even if you cannot find a full party grab who you can and queue up together so at least you know of two or more people who are not going to be terrible or burden the group. Most groups you can tell by 25% in your chances of taking down the boss, and if you see someone doing something crazy, TELL THEM. If no one lets them know they will assume they are doing a good job, which makes it worse on the next group.