Because no one ever knows where the highest price will be and where the lowest price will be. Your purchases and sales are random points on the chart.
That is obvious since Bitcoin and other digital currencies are volatile, no one can predict or knows what will be the movement of the price but that is not the reason why they sell low and buy at high. People do this since it is the most common strategy or way to earn on your investments in digital currencies, that is just the reason I am seeing.