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because buying when something is trending downward is not as profitable as when it is making new highs. I know this seems counter to common advice but if you check out the CANSLIM stock selection process which made over 25% a year returns for decades it would buy as stocks were making new highs. To catch a rocket to the moon you don't get on it when it is falling to the ground but as it is taking off lol

the other thing is the price may not ever be lower than the current moment.