That works a solid 90% of the time I'm sure but sometimes being the most experienced doesn't make someone the best leader. For instance, maybe the person with the most experience hates explaining things to newcomers (or just isn't good at it), something that seems like a big part of leading a raid team with noobs on it, then I wouldn't want them leading unless they were actually the ONLY person with experience. They don't really want to, so why make/let them?