The differences between matchmaking and looking for people online are small but incredibly important. They key differences being. In matchmaking you are paired randomly with players who A) may not be willing to talk or party up. B) may not have appropriate gear. C) could all be the same class or be running a subclass that is useless in the raid. When looking online you can ensure none of this happens, but that is not the main issues the main issue is when people matchmake an incredibly difficult experience, like a raid, and A, B, or C happens it can lead to a wipe, and usually does. When wiping over matchmaking happens too much people begin to complain about the experience being too difficult, when this happens a lot the experience gets nerfed and ruined for those who wanted the challenge. It would be easier and nicer, but in the end it would ruin raiding like it did in WoW.