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.
No it is not right bitcoin price can be known using the signature campaign and the bitcoin talk as there we use to discuss about the price of the bitcoin also we can get to know about the high and the low price of bitcoin through the market value rise and fall price of bitcoin use to increase when the market will be green and price decreased when the market will be red, so I think proper planning is required to avoid this kind of situation.