There are many people that do run these quests at level for xp, it's just that instead of XP sacking Wizard King which could be 10+ runs and an hour or two of time dependent on the group, you have one run at something like Reaver for 10 minutes or so of time, so you just don't spend as much time in the raid as you do while farming any other quest of the same level. That Reaver run could be 50,000 xp for one run but because of the timer, you'll do your one run then move on to WK anyway.