Your assessment assumes a perfectly rational, 100% transparent market. The market obviously has a distribution of rationality and transparency. The most efficient parts of the market are going to be the parts that are largely rational and/or highly transparent. Making money in the remaining more irrational/opaque parts of the market is not as simple as just identifying it as inefficient, it requires either finding a pattern in the irrationality or a way of unlocking hidden information(ideally legally).