Vote diversity: The most given reason for the limit is that it ensures that it takes many people to deal with the queue. We don't want one person to deal with everything because there's no balance in opinion or method.
Fresh meatbags: If somebody were allowed to [heroically] hog all the reviews, that also stops new users entering into servitude. People leave and others find their time restricted. We need a fluid review team and if the same old crew are dealing with everything, others won't have a chance to get into the flow.
Personal sustainability: We also don't want people to burn out. This isn't a sprint. Speaking as somebody who has on occasion spent too long on the site in a day (for an entire month), I can tell you it takes a while to recover from that sort of investment. We'd rather more people gave less but more consistently.