I think you're wildly overestimating the rationality of investors.
If other investors are irrational that's a wonderful arbitrage opportunity. But good arbitrage opportunities are rare and don't last very long, so maybe they are rational.
If you're smarter than other investors, you can make a lot of money doing the opposite of what they do.
If other investors are irrational that's a wonderful arbitrage opportunity. But good arbitrage opportunities are rare and don't last very long, so maybe they are rational.