Psychological reasons. Retail traders performance chase, selling things when they have temporary (multi-year) losses and buy when they have temporary high gains

Psychological reasons. Retail traders performance chase, selling things when they have temporary (multi-year) losses and buy when they have temporary high gains. It's incredible how strong it is even for people who are somewhat knowledgeable about finance like Bogleheads. It's also been studied by Jack Bogle himself. There was a famous study comparing Value vs. Growth stocks he noted where retail traders who invested based off either ended up doing worse than just holding both because in aggregate they kept buying the one that recently did well and then selling it when it had done poor. Buy high, sell low.