The weird thing is, most people who sell lose money because in the long run only the holders win money by the looks of it. How? Well let's say there are people who constantly buy and sell, and they rarely hold bitcoin, only thing they do is buy or sell and keep doing that and keep their money in fiat, and there are people who buy and hold, that means people who hold will always be in profit and people who bought and sold will be in a loss even if not real loss it would be loss or potential profit...
I think there are 2 reasons for this, one of which is that the trader sells BTC in the hope of buying it at a lower price and making a profit on the price difference. The second reason is that there is a lack of confidence in the stability of bitcoin and the investor is afraid of its depreciation, as it was in 2017.