Nobody believes in the 100% efficient market. It's not even possible.
But if someone's got a bead on a 99.99% efficient market, they can make a lot of money, whereas if you say "Ah ha! The market is not perfectly efficient! Here, I can make money doing this!", you probably won't make much.
(Then there are those who think the market is like maybe 10% efficient, in which case my response is, if it's so inefficient, go make money. That would imply all sorts of arbitrage opportunity. Why slave away in whatever job you may have now when there's such an inefficient market just waiting for someone smart & clever to pluck it? The answer is that where ever the market may be on the range of 0-100% efficient, to the extent that such a measurement is even particularly definable, while it may not be exactly 100.00000%, it's much closer to that than the 0% side.)
(Remember, efficient doesn't mean good or moral, which I think is the most common mistake I see on HN. It means something much more like "there are no easy arbitrage possibilities". It hasn't got anything to do with "goodness" or "morality" any more than analyzing gravity or electromagnetism in terms of its goodness or morality makes any sense.)
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).
But if someone's got a bead on a 99.99% efficient market, they can make a lot of money, whereas if you say "Ah ha! The market is not perfectly efficient! Here, I can make money doing this!", you probably won't make much.
(Then there are those who think the market is like maybe 10% efficient, in which case my response is, if it's so inefficient, go make money. That would imply all sorts of arbitrage opportunity. Why slave away in whatever job you may have now when there's such an inefficient market just waiting for someone smart & clever to pluck it? The answer is that where ever the market may be on the range of 0-100% efficient, to the extent that such a measurement is even particularly definable, while it may not be exactly 100.00000%, it's much closer to that than the 0% side.)
(Remember, efficient doesn't mean good or moral, which I think is the most common mistake I see on HN. It means something much more like "there are no easy arbitrage possibilities". It hasn't got anything to do with "goodness" or "morality" any more than analyzing gravity or electromagnetism in terms of its goodness or morality makes any sense.)